Shared MIME-info Database
2 Overview of previous systems #
2.1 KDE #
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] 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 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
2.2 GNOME #
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' extension
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:
application/msword description=Microsoft Word document [de]description=Microsoft Word-Dokument ...
Guidelines for writing descriptions can be found in the
mime-descriptions-guidelines.txt file.
The format for magic entries is defined as:
# The format of magic entries is: # # offset_start[:offset_end] pattern_type pattern [&pattern_mask] type # # <offset_start> and <offset_end> are decimal numbers (file offsets). # # <pattern_type> is (byte | short | long | string | date | beshort | # belong | bedate | leshort | lelong | ledate). # # <pattern> is an ASCII string with non-printable characters escaped # as hex or octal escape sequences, and spaces and other important # whitespace escaped with '\'. # # <pattern_mask> is a string of hex digits. The mask must be the same # length as the pattern. # # <type> is a valid MIME type. # # Order magic patterns such that ambiguous ones (such as # application/x-ms-dos-executable) are at the end of the list and # therefore get applied last. # # Avoid rules that require a seek deep into the examined file. If you # must, locate such rules at the end of the list so that they get # applied last # # When designing new document formats, make them easily recognizable # by defining a sufficiently unique magic pattern near the document # start. A good pattern is at least four bytes long and contains one # or two non-printable characters so that text files won't be # misidentified.
2.3 ROX #
Note that ROX is now using this specification. This section details the previous implementation.
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:
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.