Shared MIME-info Database

Table of Contents

What is this spec?
Language used in this specification
Overview of previous systems
Unified system
File format
Pattern matching
Contents matching
Dealing with conflicts
Directory layout
Security implications



This is version 0.5 of the Shared MIME-info Database spec, last updated 25 April 2002.

What is this spec?

Many programs and desktops use the MIME system to represent the types of files. Frequently, it is necessary to work out the correct MIME type for a file. This is generally done by examining the file's name or contents, and looking up the correct MIME type in a database.

For interoperability, it is useful for different programs to use the same database so that different programs agree on the type of a file and new rules for determining the type apply to all programs.

This specification attempts to unify the type-guessing systems currently in use by GNOME, KDE and ROX. Only the name-to-type and contents-to-type mappings are covered by this spec; other MIME type information, such as the default handler for a particular type, or the icon to use to display it in a file manager, are not covered since these are a matter of style.

Language used in this specification

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119.

Overview of previous systems


KDE uses .desktop files, with Type=MimeType, one file per type to determine type from file name. The files are arranged in the filesystem to mirror the two-level MIME type hierarchy. The syntax is very similar to other .desktop files, with Name=, Comment= etc.

Example file:

[Desktop Entry]
[... etc. other translations ]


KDE does not have a separate system for specifying extension matches, but uses case-sensitive glob patterns for everything.

A single file stores all the rules for recognising files by content. This is almost identical to file(1)'s magic.mime database file, but without the encoding field.

The format is described in the file itself as follows:

# The format is 4-5 columns:
#    Column #1: byte number to begin checking from, ">" indicates continuation
#    Column #2: type of data to match
#    Column #3: contents of data to match
#    Column #4: MIME type of result


GNOME uses the gnome-vfs library to determine the MIME type of a file. This library loads name-to-type rules from files with a '.mime' extenstion in a system-wide directory (set at install time), and merged with those in the user's directory. It loads textual descriptions for the types from files in the same directories, ending with '.keys'. The file gnome-vfs.mime in the system directory is always loaded first (allowing everything else to override it). The file user.mime in the user's directory is always loaded last, making these settings take precedence over all others.

The format of the .mime files are described as follows:

# Mime types as provided by the GNOME libraries for GNOME.
# Applications can provide more mime types by installing other
# .mime files in the PREFIX/share/mime-info directory.
# The format of this file is:
# mime-type
#	ext[,prio]:	list of extensions for this mime-type
#	regex[,prio]:	a regular expression that matches the filename
# more than one ext: and regex: fields can be present.
# prio is the priority for the match, the default is 1. This is required
# to distinguish composed filenames, for example .gz has a priority of 1
# and .tar.gz has a priority of 2 (thus a file having the filename
# something.tar.gz will match the mime-type for tar.gz before the mime-type
# for .gz
# The values in this file are kept in alphabetical order for convenience.
# Please maintain this when adding new types. Also consider adding a
# human-readable description to gnome-vfs.keys when adding a new type here.
# Also do please not add illegal mime types, observe the mime standard when
# adding new types.

When looking up the type for a file, gnome-vfs looks first for an exact-case match for the extension, then an all upper-case match, then an all lower-case match. If no matches are found, or there is no '.' in the name, then the regular expression matches are checked. It does this first for rules with priority 2, then for those with priority 1. The modification time on the mime-info directories is used to detect changes.

The .keys files contain type-to-description rules, eg:

	description=Microsoft Word document
	[de]description=Microsoft Word-Dokument

Guidelines for writing descriptions can be found in the mime-descriptions-guidelines.txt file.


ROX searches MIME-info directories in CHOICESPATH (~/Choices/MIME-info:/usr/local/share/Choices/MIME-info:/usr/share/Choices/MIME-info by default). Files from earlier directories override those in later ones, but the order within a directory is not specified.

The files are in the same format as GNOME, except:

  • There are no .keys files, so files of all extensions are loaded.

  • The priority is ignored.

  • A case-sensitive match is tried first, then a lower-case match. No upper-case match is tried.

  • Multiple extensions are allowed. Eg:

    	ext: ps.gz eps.gz

When looking up the type for a file, ROX starts with the first '.' and tries a case-sensitive match of the remaining text against the extensions. The it tries again with the filename in lower-case. It then tries again from the second '.', and so on. If no type is found, it tries the regular expressions.

ROX has no rules for determining a file's type from its contents.

Unified system

In discussions about these systems, 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 spec proposes:

  • A standard format for these files.

  • Standard locations for them.

File format

The new format is very similar to the KDE format. However, only the tags used in this example are valid:

[MIME-Info text/html]
Comment=HTML document
[... etc. other translations ]
Contents=(starts-with "<HTML")

The entries in Patterns are separated by semicolons. There is no trailing semicolon.

Specifically, all KDE-specific tags have been removed, as well as the Icon field. Although all desktops need a way to determine the icon for a particular type, the icon used will depend on desktop, and not only on the file type. The type should be a standard MIME type where possible. If a special media type is required for non-file objects (directories, pipes, etc), then the media type 'inode' may be used.

Although not part of the name-to-type mapping, the Comment field is left in for the sake of not having too many files. The Hidden field is usually not present. It is used to indicate that this entry replaces all information for this MIME type read so far, instead of being merged with other records for the same type. The intent is to let users entirely replace existing types.

Pattern matching

KDE's Patterns field 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.

Applications MUST first try a case-sensitive match, then a case-insensitive one. This is so that main.C will be seen as a C++ file, but IMAGE.GIF will still use the *.gif pattern.

Contents matching

The value of the Contents attribute is a scheme-like expression. If the expression evaluates to a true value then the file is assumed to be of this type. Since scanning a file's contents can be very slow, applications may choose to do pattern matching first and only fallback to content matching, or not perform it at all.

An expression is a list of space-separated items surrounded by parenthesis, eg:

(starts-with "<?xml ")

The first element of the list (starts-with in this example) is the name of a function. The remaining elements are its arguments. The result of evaluating the expression is the result of applying the function to the arguments. Each argument may be:

An integer

A 64-bit signed integer, such as 32.

A string

A string of characters with C-style escaping. This string contains the sequence of bytes <0, 8, 9, 10>: "\0\010\t\xa".

A symbol

A symbol is a constant for the file being tested. For example, size evaluates to the file's size.

A list

Lists may be nested. Each sub-list is evaluated in the same way as the top-level list, eg (+ (* 3 2) (* 4 3)) is 18.

Functions may return integers or strings. 'True' is represented by the integer 1, and False by 0. The following functions and symbols are provided:

Function exampleResultDescription
(+ 1 2 3)6The sum of the arguments
(- 10 6 6)-2The first argument minus the sum of the remaining arguments
(* 2 2 3)12The product of the arguments
(/ 20 2 2)5The first argument divided by the product of the remaining arguments.
(> 1 2)0True iff the first aguement is greater than the second
(< 1 2)1True iff the first aguement is less than the second
(= 1 2)0True iff the first aguement is equal to the second
size32The size of the file in bytes
(starts-with "GIF89a")1True iff the file starts with the given sequence of bytes
(not size)1True iff argument is false (0 or "")
(and "one" 2 3)3The first false argument, or the last argument if none are false.
(or 0 "" 2 0)2The first true argument, or the last argument if none are true.

and and or should only evaluate as many arguments as are necessary to determine the result.

Dealing with conflicts

If several patterns 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 acceptable to match patterns of the form '*.text' before other wildcarded patterns (that is, to special-case extensions using a hash table).

If the same pattern is defined twice, then they SHOULD be ordered by the directory the rule came from (this is to allow users to override the system defaults if, for example, they are using a common extension to mean something else). If they came from the same directory, either can be used.

If the same type is defined in several places, the Patterns and Comments MUST be merged. If two different comments are provided for the same MIME type in the same language, they should be ordered by directory as before.

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

Directory layout

Unlike the KDE system, the files are not arranged in the filesystem by type. This approach is only possible for a tightly coordinated system. Consider, for example, that ROX-Filer adds a mapping from .DirIcon to 'image/png'. This cannot be specified in a file called image/png.desktop without conflicting with existing definitions for the type.

Since files are not named by type, each file may contain multiple types. The files should be named by the package that they come from to avoid conflicts and reduce loading times.

The directories to be used to load these files are:

  • /usr/share/mime/mime-info

  • /usr/local/share/mime/mime-info

  • ~/.mime/mime-info

Each of these directories contains a number of files with the '.mimeinfo' extension. Applications MUST NOT load other files. This is to allow for future extensions.

Programs modifying any of these files MUST update the modification time on the parent (mime-info) directory so that applications can easily detect the change. The rules from the directories in this list take precedence over conflicting rules from earlier directories. Thus, the user's settings take precedence over all others.

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.