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:

File format

The new format is very similar to that described in the Desktop Entries Specification[DesktopEntries]. However, only the tags used in this example are valid:

[MIME-Info text/html]
Comment=HTML document
[... etc. other translations ]
Contents=50:(string 0:64 "<HTML")

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 Encoding tag is not present; all .mimeinfo files are in the UTF-8 encoding.

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.

The entries in Patterns are separated by semicolons. There is no trailing semicolon. PreferredExtension is the suggested extension to use when creating files of this type.

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.

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 instead 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 try to 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. If a directory contains a file called user.mimeinfo then it should be read after all other files in that directory. This is to allow the user's settings to take precedence over all others. GUI tools for editing the MIME types will edit ~/.mime/mime-info/user.mimeinfo.

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.

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 MUST 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). Patterns in ~/.mime/mime-info override those in /usr/local/share/mime/mime-info, which in turn take precedence over those from /usr/mime/mime-info. If a pattern is defined twice within 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 types, conflicts should be rare.

Contents matching

The value of the Contents attribute contains a priority and an expression. If several expressions match for one file, the one with the highest priority is used. As a guide, priorities should be between 1 and 100, with 50 being the normal case. Generic types (such as XML or GZip-compressed files) should have lower priorities.

Since scanning a file's contents can be very slow, applications may choose to do pattern matching first and only fall back to content matching, or not perform it at all.

The basic building blocks of expressions are bracketed lists containing a type, an offset (or range of offsets), the data to match and, optionally, a mask. For example:

(string 0 "%PDF-")
(string 0 "\177ELF")
(string	0:64 "<svg")
(string 0 "BMxxxx\000\000" 0xffff00000000ffff)

The first element of the list is the type of the data (see the table below), the second is the range of offsets to check, the third is the value to match and the last, if present, is the mask.

Integers have the usual C-style prefixes (0 for octal numbers, 0x for hexadecimal). Strings have C-style escaping. This string contains the sequence of bytes <0, 8, 9, 10>: "\0\010\t\xa".

A range gives the range of valid starting offsets. If the end of the range is omitted then it is assumed to be the same as the start (that is, the match is only checked at one point in the file).

The possible types of match are listed below:

stringString of bytes 
byteSingle byte 
big1616-bit big-endian integer 
big3232-bit big-endian integer 
little1616-bit little-endian integer 
little3232-bit little-endian integer 
host1616-bit integer in host-order 
host3232-bit integer in host-order 

These basic expressions may be combined using the and and or syntax, eg:

(and (string 0 "\037\213") (string 10 "KOffice") (string 18 "application/x-kchart\004\006"))

The and keyword corresponds to a more-deeply indented continuation line in the original file(1) syntax, while or corresponds to elements at the same indentation. They may be nested in the obvious (scheme-like) fashion.

Since many formats have sub-formats (for example, KOffice stores its files in GZip format, with a generic KOffice marker and a specific application marker), it may be a useful optimisation to spot the same subexpression (eg (string 10 "KOffice")) being used in several types and only check it once.

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.