Table of Contents
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.
.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
with Name=, Comment= etc.
[Desktop Entry] Encoding=UTF-8 MimeType=application/x-kword Comment=KWord Comment[af]=kword [... etc. other translations ] Icon=kword Type=MimeType Patterns=*.kwd;*.kwt; X-KDE-AutoEmbed=false [Property::X-KDE-NativeExtension] Type=QString Value=.kwd
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
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, 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
directories is used to detect changes.
The .keys files contain type-to-description rules, eg:
application/msword description=Microsoft Word document [de]description=Microsoft Word-Dokument ...
Guidelines for writing descriptions can be found in the
MIME-info directories in
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:
application/x-compressed-postscript 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.
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.
A standard location for them.
The new format is very similar to the KDE format. However, only the tags used in this example are valid:
[MIME-Info text/html] Encoding=UTF-8 Comment=HTML document Comment[af]=... [... etc. other translations ] Patterns=*.htm;*.html; Contents=(starts-with "<HTML") Hidden=false
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.
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,
IMAGE.GIF will still use the *.gif pattern.
The value of the Contents attribute is a scheme 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.
This is just a vague proposal at the moment. Also, need a list of functions to provide.
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.
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:
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.
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).