Shared MIME-info Database
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# 2. Unified system

In discussions about the previous systems used by GNOME, KDE and ROX (see the "History and related systems" document), it was clear that the differences between the databases were simply a result of them being separate, and not due to any fundamental disagreements between developers. Everyone is keen to see them merged.

This specification proposes:

- A standard way for applications to install new MIME related information.

- A standard way of getting the MIME type for a file.

- A standard way of getting information about a MIME type.

- Standard locations for all the files, and methods of resolving conflicts.

Further, the existing databases have been merged into a single package [SharedMIME].

## 2.1. Directory layout

There are two important requirements for the way the MIME database is stored:

- Applications must be able to extend the database in any way when they are installed, to add both new rules for determining type, and new information about specific types.

- It must be possible to install applications in /usr, /usr/local and the user's home directory (in the normal Unix way) and have the MIME information used.

This specification uses the XDG Base Directory Specification[BaseDir] to define the prefixes below which the database is stored. In the rest of this document, paths shown with the prefix `<MIME>` indicate the files should be loaded from the `mime` subdirectory of every directory in `XDG_DATA_HOME`:`XDG_DATA_DIRS`.

For example, when using the default paths, "Load all the `<MIME>/text/html.xml` files" means to load `/usr/share/mime/text/html.xml`, `/usr/local/share/mime/text/html.xml`, and `~/.local/share/mime/text/html.xml` (if they exist, and in this order). Information found in a directory is added to the information found in previous directories, except when `glob-deleteall` or `magic-deleteall` is used to overwrite parts of a mimetype definition.

Each application that wishes to contribute to the MIME database will install a single XML file, named after the application, into one of the three `<MIME>/packages/` directories (depending on where the user requested the application be installed). After installing, uninstalling or modifying this file, the application MUST run the update-mime-database command, which is provided by the freedesktop.org shared database[SharedMIME].

update-mime-database is passed the `mime` directory containing the `packages` subdirectory which was modified as its only argument. It scans all the XML files in the `packages` subdirectory, combines the information in them, and creates a number of output files.

Where the information from these files is conflicting, information from directories lower in the list takes precedence. Any file named `Override.xml` takes precedence over all other files in the same `packages` directory. This can be used by tools which let the user edit the database to ensure that the user's changes take effect.

The files created by update-mime-database are:

- `<MIME>/globs` (contains a mapping from names to MIME types) [deprecated for globs2]

- `<MIME>/globs2` (contains a mapping from names to MIME types and glob weight)

- `<MIME>/magic` (contains a mapping from file contents to MIME types)

- `<MIME>/subclasses` (contains a mapping from MIME types to types they inherit from)

- `<MIME>/aliases` (contains a mapping from aliases to MIME types)

- `<MIME>/icons` (contains a mapping from MIME types to icons)

- `<MIME>/generic-icons` (contains a mapping from MIME types to generic icons)

- `<MIME>/XMLnamespaces` (contains a mapping from XML (namespaceURI, localName) pairs to MIME types)

- `<MIME>/MEDIA/SUBTYPE.xml` (one file for each MIME type, giving details about the type, including comment, icon and generic-icon)

- `<MIME>/mime.cache` (contains the same information as the `globs2`, `magic`, `subclasses`, `aliases`, `icons`, `generic-icons` and `XMLnamespaces` files, in a binary, mmappable format)

The format of these generated files and the source files in `packages` are explained in the following sections. This step serves several purposes. First, it allows applications to quickly get the data they need without parsing all the source XML files (the base package alone is over 700K). Second, it allows the database to be used for other purposes (such as creating the `/etc/mime.types` file if desired). Third, it allows validation to be performed on the input data, and removes the need for other applications to carefully check the input for errors themselves.

## 2.2. The source XML files

Each application provides only a single XML source file, which is installed in the `packages` directory as described above. This file is an XML file whose document element is named `mime-info` and whose namespace URI is http://www.freedesktop.org/standards/shared-mime-info. All elements described in this specification MUST have this namespace too.

The document element may contain zero or more `mime-type` child nodes, in any order, each describing a single MIME type. Each element has a `type` attribute giving the MIME type that it describes.

Each `mime-type` node may contain any combination of the following elements, and in any order:

- `glob` elements have a `pattern` attribute. Any file whose name matches this pattern will be given this MIME type (subject to conflicting rules in other files, of course). There is also an optional `weight` attribute which is used when resolving conflicts with other glob matches. The default weight value is 50, and the maximum is 100.

  KDE's glob system replaces GNOME's and ROX's ext/regex fields, since it is trivial to detect a pattern in the form '*.ext' and store it in an extension hash table internally. The full power of regular expressions was not being used by either desktop, and glob patterns are more suitable for filename matching anyway.

  The first glob element represents the "main" extension for the file type. While this doesn't affect the mimetype matching algorithm, this information can be useful when a single main extension is needed for a mimetype, for instance so that applications can choose an appropriate extension when saving a file.

- A `glob-deleteall` element, which indicates that patterns from previously parsed directories must be discarded. The patterns defined in this file (if any) are used instead.

- `magic` elements contain a list of `match` elements, any of which may match, and an optional `priority` attribute for all of the contained rules. Low numbers should be used for more generic types (such as 'gzip compressed data') and higher values for specific subtypes (such as a word processor format that happens to use gzip to compress the file). The default priority value is 50, and the maximum is 100.

  Each `match` element has a number of attributes:

  Attribute | Required? | Value
  --- | --- | ---
  type | Yes | `string`, `host16`, `host32`, `big16`, `big32`, `little16`, `little32` or `byte`.
  offset | Yes | The byte offset(s) in the file to check. This may be a single number or a range in the form `start:end', indicating that all offsets in the range should be checked. The range is inclusive.
  value | Yes | The value to compare the file contents with, in the format indicated by the type attribute. The string type supports the C character escapes (\0, \t, \n, \r, \xAB for hex, \777 for octal).
  mask | No | The number to AND the value in the file with before comparing it to `value'. Masks for numerical types can be any number, while masks for strings must be in base 16, and start with 0x.

  Each element corresponds to one line of file(1)'s `magic.mime` file. They can be nested in the same way to provide the equivalent of continuation lines. That is, <a><b/><c/></a> means 'a and (b or c)'.

- A `magic-deleteall` element, which indicates that magic matches from previously parsed directories must be discarded. The magic defined in this file (if any) is used instead.

- `alias` elements indicate that the type is also sometimes known by another name, given by the `type` attribute. For example, `audio/midi` has an alias of `audio/x-midi`. Note that there should not be a `mime-type` element defining each alias; a single element defines the canonical name for the type and lists all its aliases.

- `sub-class-of` elements indicate that any data of this type is also some other type, given by the `type` attribute. See Section 2.11.

- `comment` elements give a human-readable textual description of the MIME type, usually composed of an acronym of the file name extension and a short description, like "ODS spreadsheet". There may be many of these elements with different `xml:lang` attributes to provide the text in multiple languages.

- `acronym` elements give experienced users a terse idea of the document contents. for example "ODS", "GEDCOM", "JPEG" and "XML".

- `expanded-acronym` elements are the expanded versions of the acronym elements, for example "OpenDocument Spreadsheet", "GEnealogical Data COMmunication", and "eXtensible Markup Language". The purpose of these elements is to provide users a way to look up information on various MIME types or file formats in third-party resources.

- `icon` elements specify the icon to be used for this particular mime-type, given by the `name` attribute. Generally the icon used for a mimetype is created based on the mime-type by mapping "/" characters to "-", but users can override this by using the `icon` element to customize the icon for a particular mimetype. This element is not used in the system database, but only used in the user overridden database. Only one `icon` element is allowed.

- `generic-icon` elements specify the icon to use as a generic icon for this particular mime-type, given by the `name` attribute. This is used if there is no specific icon (see `icon` for how these are found). These are used for categories of similar types (like spreadsheets or archives) that can use a common icon. The Icon Naming Specification lists a set of such icon names. If this element is not specified then the mimetype is used to generate the generic icon by using the top-level media type (e.g. "video" in "video/ogg") and appending "-x-generic" (i.e. "video-x-generic" in the previous example). Only one `generic-icon` element is allowed.

- `root-XML` elements have `namespaceURI` and `localName` attributes. If a file is identified as being an XML file, these rules allow a more specific MIME type to be chosen based on the namespace and localname of the document element.

  If `localName` is present but empty then the document element may have any name, but the namespace must still match.

- `treemagic` elements contain a list of `treematch` elements, any of which may match, and an optional `priority` attribute for all of the contained rules. The default priority value is 50, and the maximum is 100.

  Each `treematch` element has a number of attributes:

  Attribute | Required? | Value
  --- | --- | ---
  path | Yes | A path that must be present on the mounted volume/filesystem. The path is interpreted as a relative path starting at the root of the tested volume/filesystem
  type | No | The type of path. Possible values: `file`, `directory`, `link`
  match-case | No | Whether path should be matched case-sensitively. Possible values: `true`, `false`
  executable | No | Whether the file must be executable. Possible values: `true`, `false`
  non-empty | No | Whether the directory must be non-empty. Possible values: `true`, `false`
  mimetype | No | The mimetype for the file at path

  `treematch` elements can be nested, meaning that both the outer and the inner `treematch` must be satisfied for a "match".

Applications may also define their own elements, provided they are namespaced to prevent collisions. Unknown elements are copied directly to the output XML files like `comment` elements. A typical use for this would be to indicate the default handler application for a particular desktop ("Galeon is the GNOME default text/html browser"). Note that this doesn't indicate the user's preferred application, only the (fixed) default.

Here is an example source file, named `diff.xml`:

    <?xml version="1.0"?>
    <mime-info xmlns='http://www.freedesktop.org/standards/shared-mime-info'>
      <mime-type type="text/x-diff">
        <comment>Differences between files</comment>
        <comment xml:lang="af">verskille tussen lêers</comment>
        ...
        <magic priority="50">
          <match type="string" offset="0" value="diff\t"/>
          <match type="string" offset="0" value="***\t"/>
          <match type="string" offset="0" value="Common subdirectories: "/>
        </magic>
        <glob pattern="*.diff"/>
        <glob pattern="*.patch"/>
      </mime-type>
    </mime-info>

In practice, common types such as text/x-diff are provided by the freedesktop.org shared database. Also, only new information needs to be provided, since this information will be merged with other information about the same type.

## 2.3. The MEDIA/SUBTYPE.xml files

These files have a `mime-type` element as the root node. The format is as described above. They are created by merging all the `mime-type` elements from the source files and creating one output file per MIME type. Each file may contain information from multiple source files. The `magic`, `glob` and `root-XML` elements will have been removed.

The example source file given above would (on its own) create an output file called `<MIME>/text/x-diff.xml` containing the following:

    <?xml version="1.0" encoding="utf-8"?>
    <mime-type xmlns="http://www.freedesktop.org/standards/shared-mime-info" type="text/x-diff">
    <!--Created automatically by update-mime-database. DO NOT EDIT!-->
      <comment>Differences between files</comment>
      <comment xml:lang="af">verskille tussen lêers</comment>
      ...
    </mime-type>

## 2.4. The glob files

The globs2 file is a simple list of lines containing weight, MIME type and pattern, separated by a colon. The lines are ordered by glob weight. For example:

    # This file was automatically generated by the
    # update-mime-database command. DO NOT EDIT!
    ...
    55:text/x-diff:*.patch
    50:text/x-diff:*.diff
    50:text/x-c++src:*.C:cs
    ...

The glob file is a simple list of lines containing a MIME type and pattern, separated by a colon. It is deprecated in favour of the globs2 file which also lists the weight of the glob rule. The lines are ordered by glob weight. For example:

    # This file was automatically generated by the
    # update-mime-database command. DO NOT EDIT!
    ...
    text/x-diff:*.patch
    text/x-diff:*.diff
    ...

Applications MUST match globs case-insensitively, except when the case-sensitive attribute is set to true. This is so that e.g. `main.C` will be seen as a C++ file, but `IMAGE.GIF` will still use the *.gif pattern.

If several patterns of the same weight match then the longest pattern SHOULD be used. In particular, files with multiple extensions (such as `Data.tar.gz`) MUST match the longest sequence of extensions (eg '*.tar.gz' in preference to '*.gz'). Literal patterns (eg, 'Makefile') must be matched before all others. It is suggested that patterns beginning with `*.' and containing no other special characters (`*?[') should be placed in a hash table for efficient lookup, since this covers the majority of the patterns. Thus, patterns of this form should be matched before other wildcarded patterns.

If a matching pattern is provided by two or more MIME types, applications SHOULD not rely on one of them. They are instead supposed to use magic data (see below) to detect the actual MIME type. This is for instance required to deal with container formats like Ogg or AVI, that map various video and/or audio-encoded data to one extension.

There may be several rules mapping to the same type. They should all be merged. If the same pattern is defined twice, then they MUST be ordered by the directory the rule came from, as described above.

The `glob-deleteall` element, which means that implementations SHOULD discard information from previous directories, is written out into the globs2 file using __NOGLOBS__ as the pattern. For instance:

    0:text/x-diff:__NOGLOBS__
    50:text/x-diff:*.diff
    ...

In the above example, the mimetype text/x-diff is redefined (for instance in a user's ~/.local/share/mime) to only be associated with the pattern *.diff, so the other patterns like *.patch were removed. The weight in front of the __NOGLOBS__ line is ignored. In a given globs2 file, the __NOGLOBS__ line for a given mimetype is always written out before any other globs for this mimetype.

Lines beginning with `#' are comments and should be ignored. Everything from the `:' character to the newline is part of the pattern; spaces should not be stripped. The file is in the UTF-8 encoding. The format of the glob pattern is as for fnmatch(3). The format does not allow a pattern to contain a literal newline character, but this is not expected to be a problem.

Common types (such as MS Word Documents) will be provided in the X Desktop Group's package, which MUST be required by all applications using this specification. Since each application will then only be providing information about its own types, conflicts should be rare.

The fourth field ("cs" in the first globs2 example) contains a list of comma-separated flags. The flags currently defined are: cs (for case-sensitive). Implementations should ignore unknown flags.

Implementations should also ignore further fields, so that the syntax of the globs2 file can be extended in the future. Example: "50:text/x-c++src:*.C:cs,newflag:newfeature:somethingelse" should currently be parsed as "50:text/x-c++src:*.C:cs".

## 2.5. The magic files

The magic data is stored in a binary format for ease of parsing. The old magic database had complex escaping rules; these are now handled by update-mime-database.

The file starts with the magic string "MIME-Magic\0\n". There is no version number in the file. Incompatible changes will be handled by creating both the current `magic' file and a newer `magic2' in the new format. Where possible, compatible changes only will be made. All numbers are big-endian, so need to be byte-swapped on little-endian machines.

The rest of the file is made up of a sequence of small sections. Each section is introduced by giving the priority and type in brackets, followed by a newline character. Higher priority entries come first. Example:

    [50:text/x-diff]\n

Each line in the section takes the form:

    [ indent ] ">" start-offset "=" value
    [ "&" mask ] [ "~" word-size ] [ "+" range-length ] "\n"

Part | Example | Meaning
--- | --- | ---
indent | 1 | The nesting depth of the rule, corresponding to the number of '>' characters in the traditional file format.
">" start-offset | >4 | The offset into the file to look for a match.
"=" value | =\0x0\0x2\0x55\0x40 | Two bytes giving the (big-endian) length of the value, followed by the value itself.
"&" mask | &\0xff\0xf0 | The mask, which (if present) is exactly the same length as the value.
"~" word-size | ~2 | On little-endian machines, the size of each group to byte-swap.
"+" range-length | +8 | The length of the region in the file to check.

Note that the value, value length and mask are all binary, whereas everything else is textual. Each of the elements begins with a single character to identify it, except for the indent level.

The word size is used for byte-swapping. Little-endian systems should reverse the order of groups of bytes in the value and mask if this is greater than one. This only affects `host' matches (`big32' entries still have a word size of 1, for example, because no swapping is necessary, whereas `host32' has a word size of 4).

The indent, range-length, word-size and mask components are optional. If missing, indent defaults to 0, range-length to 1, the word-size to 1, and the mask to all 'one' bits.

Indent corresponds to the nesting depth of the rule. Top-level rules have an indent of zero. The parent of an entry is the preceding entry with an indent one less than the entry.

If an unknown character is found where a newline is expected then the whole line should be ignored (there will be no binary data after the new character, so the next line starts after the next "\n" character). This is for future extensions.

The text/x-diff above example would (on its own) create this magic file:

    00000000  4d 49 4d 45 2d 4d 61 67  69 63 00 0a 5b 35 30 3a  |MIME-Magic..[50:|
    00000010  74 65 78 74 2f 78 2d 64  69 66 66 5d 0a 3e 30 3d  |text/x-diff].>0=|
    00000020  00 05 64 69 66 66 09 0a  3e 30 3d 00 04 2a 2a 2a  |..diff..>0=..***|
    00000030  09 0a 3e 30 3d 00 17 43  6f 6d 6d 6f 6e 20 73 75  |..>0=..Common su|
    00000040  62 64 69 72 65 63 74 6f  72 69 65 73 3a 20 0a     |bdirectories: .|

The `magic-deleteall` attribute, which means that implementations SHOULD discard information from previous directories, is written out into the magic file using __NOMAGIC__ as the value:

    >0=__NOMAGIC__\n

This can be followed by other magic rules for the mimetype.

## 2.6. The XMLnamespaces files

Each `XMLnamespaces` file is a list of lines in the form:

    namespaceURI " " localName " " MIME-Type "\n"

For example:

    http://www.w3.org/1999/xhtml html application/xhtml+xml

The lines are sorted (using strcmp in the C locale) and there are no lines with the same namespaceURI and localName in one file. If the localName was empty then there will be two spaces following the namespaceURI.

## 2.7. The icon files

The `icons` and `generic-icons` files are list of lines in the form:

    MIME-Type ":" icon-name "\n"

For example:

    application/msword:x-office-document

## 2.8. The treemagic files

The tree magic data is stored in a file with a format that is very similar to the magic file format.

The file starts with the magic string "MIME-TreeMagic\0\n". There is no version number in the file. Incompatible changes will be handled by creating both the current `treemagic' and a newer `treemagic2' in the new format. Where possible, changes will be made in a compatible fashion.

The rest of the file is made up of a sequence of small sections. Each section is introduced by giving the priority and type in brackets, followed by a newline character. Higher priority entries come first. Example:

    [50:x-content/image-dcf]\n

Each line in the section takes the form:

    [ indent ] ">" "\"" path "\"" "=" type [ "," option ]* "\n"

Part | Meaning
--- | ---
indent | The nesting depth of the rule.
path | The path to match.
type | The required file type, one of "file", "directory", "link" or "any"
option | Optional for the optional attributes of `treematch` elements. Possible values are "executable", "match-case", "non-empty", or a MIME type

## 2.9. The mime.cache files

The `mime.cache` files contain the same information as the `globs2`, `magic`, `subclasses`, `aliases` and `XMLnamespaces` files, in a binary, mmappable format:

    Header:
    2			CARD16		MAJOR_VERSION	1
    2			CARD16		MINOR_VERSION	2
    4			CARD32		ALIAS_LIST_OFFSET
    4			CARD32		PARENT_LIST_OFFSET
    4			CARD32		LITERAL_LIST_OFFSET
    4			CARD32		REVERSE_SUFFIX_TREE_OFFSET
    4			CARD32		GLOB_LIST_OFFSET
    4			CARD32		MAGIC_LIST_OFFSET
    4			CARD32		NAMESPACE_LIST_OFFSET
    4			CARD32		ICONS_LIST_OFFSET
    4			CARD32		GENERIC_ICONS_LIST_OFFSET

    AliasList:
    4			CARD32		N_ALIASES
    8*N_ALIASES		AliasListEntry

    AliasListEntry:
    4			CARD32		ALIAS_OFFSET
    4			CARD32		MIME_TYPE_OFFSET

    ParentList:
    4			CARD32		N_ENTRIES
    8*N_ENTRIES		ParentListEntry

    ParentListEntry:
    4			CARD32		MIME_TYPE_OFFSET
    4			CARD32		PARENTS_OFFSET

    Parents:
    4			CARD32		N_PARENTS
    4*N_PARENTS		CARD32		MIME_TYPE_OFFSET

    LiteralList:
    4			CARD32		N_LITERALS
    12*N_LITERALS		LiteralEntry

    LiteralEntry:
    4			CARD32		LITERAL_OFFSET
    4			CARD32		MIME_TYPE_OFFSET
    4			CARD32		WEIGHT in lower 8 bits
                                            FLAGS in rest:
                                            0x100 = case-sensitive

    GlobList:
    4			CARD32		N_GLOBS
    12*N_GLOBS		GlobEntry

    GlobEntry:
    4			CARD32		GLOB_OFFSET
    4			CARD32		MIME_TYPE_OFFSET
    4			CARD32		WEIGHT in lower 8 bits
                                            FLAGS in rest:
                                            0x100 = case-sensitive

    ReverseSuffixTree:
    4			CARD32		N_ROOTS
    4	 		CARD32		FIRST_ROOT_OFFSET

    ReverseSuffixTreeNode:
    4			CARD32		CHARACTER
    4			CARD32		N_CHILDREN
    4			CARD32		FIRST_CHILD_OFFSET

    ReverseSuffixTreeLeafNode:
    4			CARD32		0
    4			CARD32		MIME_TYPE_OFFSET
    4			CARD32		WEIGHT in lower 8 bits
                                            FLAGS in rest:
                                            0x100 = case-sensitive

    MagicList:
    4			CARD32		N_MATCHES
    4			CARD32		MAX_EXTENT
    4			CARD32		FIRST_MATCH_OFFSET

    Match:
    4			CARD32		PRIORITY
    4			CARD32		MIME_TYPE_OFFSET
    4			CARD32		N_MATCHLETS
    4			CARD32		FIRST_MATCHLET_OFFSET

    Matchlet:
    4			CARD32		RANGE_START
    4			CARD32		RANGE_LENGTH
    4			CARD32		WORD_SIZE
    4			CARD32		VALUE_LENGTH
    4			CARD32		VALUE_OFFSET
    4			CARD32		MASK_OFFSET (0 if no mask)
    4			CARD32		N_CHILDREN
    4			CARD32		FIRST_CHILD_OFFSET

    NamespaceList:
    4			CARD32		N_NAMESPACES
    12*N_NAMESPACES		NamespaceEntry

    NamespaceEntry:
    4			CARD32		NAMESPACE_URI_OFFSET
    4			CARD32		LOCAL_NAME_OFFSET
    4			CARD32		MIME_TYPE_OFFSET

    GenericIconsList:
    IconsList:
    4			CARD32		N_ICONS
    8*N_ICONS		IconListEntry

    IconListEntry:
    4			CARD32		MIME_TYPE_OFFSET
    4			CARD32		ICON_NAME_OFFSET

Lists in the file are sorted, to enable binary searching. The list of aliases is sorted by alias, the list of literal globs is sorted by the literal. The SuffixTreeNode siblings are sorted by character. The list of namespaces is sorted by namespace uri. The list of icons is sorted by mimetype.

Mimetypes are stored in the suffix tree by appending suffix tree leaf nodes with '\0' as character. These nodes appear at the beginning of the list of children.

All offsets are in bytes from the beginning of the file.

Strings are zero-terminated.

All numbers are in network (big-endian) order. This is necessary because the data will be stored in arch-independent directories like `/usr/share/mime` or even in user's home directories.

Cache files have to be written atomically - write to a temporary name, then move over the old file - so that clients that have the old cache file open and mmap'ed won't get corrupt data.

## 2.10. Storing the MIME type using Extended Attributes

An implementation MAY also get a file's MIME type from the `user.mime_type` extended attribute. The type given here should normally be used in preference to any guessed type, since the user is able to set it explicitly. Applications MAY choose to set the type when saving files. Since many applications and filesystems do not support extended attributes, implementations MUST NOT rely on this method being available.

## 2.11. Subclassing

A type is a subclass of another type if any instance of the first type is also an instance of the second. For example, all image/svg+xml files are also application/xml, text/plain and application/octet-stream files. Subclassing is about the format, rather than the category of the data (for example, there is no 'generic spreadsheet' class that all spreadsheets inherit from).

Some subclass rules are implicit:

- All text/* types are subclasses of text/plain.

- All streamable types (ie, everything except the inode/* types) are subclasses of application/octet-stream.

In addition to these rules, explicit subclass information may be given using the `sub-class-of` element.

Note that some file formats are also compressed files (application/x-jar files are also application/zip files). However, this is different to a case such as a compressed postscript file, which is not a valid postscript file itself (so application/x-gzpostscript does not inherit from application/postscript, because an application that can handle the latter may not cope with the former).

Some types may or may not be instances of other types. For example, a spreadsheet file may be compressed or not. It is a valid spreadsheet file either way, but only inherits from application/gzip in one case. This information cannot be represented statically; instead an application interested in this information should run all of the magic rules, and use the list of types returned as the subclasses.

Note that it is possible for a mime-type to be a `sub-class-of` an alias, for example if a temporary vendor mime-type is replaced by an official IANA mime-type.

## 2.12. Recommended checking order

Because different applications have different requirements, they may choose to use the various methods provided by this specification in any order. However, the RECOMMENDED order to perform the checks is:

- If a MIME type is provided explicitly (eg, by a ContentType HTTP header, a MIME email attachment, an extended attribute or some other means) then that should be used instead of guessing.

- Otherwise, start by doing a glob match of the filename. Keep only globs with the biggest weight. If the patterns are different, keep only globs with the longest pattern, as previously discussed. If after this, there is one or more matching glob, and all the matching globs result in the same mimetype, use that mimetype as the result.

- If the glob matching fails or results in multiple conflicting mimetypes, read the contents of the file and do magic sniffing on it. If no magic rule matches the data (or if the content is not available), use the default type of application/octet-stream for binary data, or text/plain for textual data. If there was no glob match, use the magic match as the result.

  Note: Checking the first 128 bytes of the file for ASCII control characters is a good way to guess whether a file is binary or text, but note that files with high-bit-set characters should still be treated as text since these can appear in UTF-8 text, unlike control characters.

- If any of the mimetypes resulting from a glob match is equal to or a subclass of the result from the magic sniffing, use this as the result. This allows us for example to distinguish text files called "foo.doc" from MS-Word files with the same name, as the magic match for the MS-Word file would be application/x-ole-storage which the MS-Word type inherits.

- Otherwise use the result of the glob match that has the highest weight.

There are several reasons for checking the glob patterns before the magic. First of all doing magic sniffing is very expensive as reading the contents of the files causes a lot of seeks, which is very expensive. Secondly, some applications don't check the magic at all (sometimes the content is not available or too slow to read), and this makes it more likely that both will get the same type.

Also, users can easily understand why calling their text file `README.mp3` makes the system think it's an MP3, whereas they have trouble understanding why their computer thinks `README.txt` is a PostScript file. If the system guesses wrongly, the user can often rename the file to fix the problem.

## 2.13. Non-regular files

Sometimes it is useful to assign MIME types to other objects in the filesystem, such as directories, sockets and device files. This could be useful when looking up an icon for a type, or for providing a textual description of one of these objects. The media type 'inode' is provided for this purpose, with the following types corresponding to the standard types of object found in a Unix filesystem:

inode/blockdevice
inode/chardevice
inode/directory
inode/fifo
inode/mount-point
inode/socket
inode/symlink

An inode/mount-point is a subclass of inode/directory. It can be useful when adding extra actions for these directories, such as 'mount' or 'eject'. Mounted directories can be detected by comparing the 'st_dev' of a directory with that of its parent. If they differ, they are from different devices and the directory is a mount point.

## 2.14. Content types for volumes

Traditional MIME types apply to individual files or bytestreams. It is often useful to apply the same methodologies when classifying the content of mountable volumes or filesystems. The x-content type has been introduced for this purpose. Typical examples are x-content/audio-dvd, x-content/blank-cd or x-content/image-dcf.

Matching of content types works with `treemagic` elements, which are analogous to the `magic` elements used for MIME type matching. Instead of looking for byte sequences in files, `treemagic` elements allow to look for files with certain names, permissions or mime types in a directory hierarchy.

## 2.15. URI scheme handlers

URI scheme handling (such as a movie player handling mms:// URIs, or a Podcast program handling feed:// URIs) are handled through applications handling the x-scheme-handler/foo mime-type, where foo is the URI scheme in question.

This scheme allows URI scheme handling to enjoy the same benefits as mime-type handlers, such as the ability to change the default handler, the cross-desktop support, and easier application launching.

Note that this virtual mime-type is not for listing URI schemes that an application can load files from. For example, a movie player would not list x-scheme-handler/http in its supported mime-types, but it would list x-scheme-handler/rtsp if it supported playing back from RTSP locations.

## 2.16. Security implications

The system described in this document is intended to allow different programs to see the same file as having the same type. This is to help interoperability. The type determined in this way is only a guess, and an application MUST NOT trust a file based simply on its MIME type. For example, a downloader should not pass a file directly to a launcher application without confirmation simply because the type looks `harmless' (eg, text/plain).

Do not rely on two applications getting the same type for the same file, even if they both use this system. The spec allows some leeway in implementation, and in any case the programs may be following different versions of the spec.

## 2.17. User modification

The MIME database is NOT intended to store user preferences. Users should never edit the database. If they wish to make corrections or provide MIME entries for software that doesn't provide these itself, they should do so by means of the Override.xml mentioned in Section 2.1. Information such as "text/html files need to be opened with Mozilla" should NOT go in the database.

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