9.5 MIME Types
A Multipurpose Internet Mail Extension (MIME) type identifies
the format of a file. The MIME type enables applications to read the
file. Applications such as Internet browsers and e-mail applications
use the MIME type to handle files of different types. For example, an
e-mail application can use the MIME type to detect what type of file
is attached to an e-mail.
The Nautilus file manager uses MIME types to identify the type
of a file. The file manager needs to know the MIME type of a file to
perform the following tasks:
-
Open the file in an appropriate application
-
Display a string that describes the type of file
-
Display an appropriate icon to represent the file
-
Display a list of other applications that can open the file
It is sometimes necessary to work out the correct MIME type for
a file. This is usually done by examining the file's name or contents
and looking up the correct MIME type in a database. If you add a new
application (that is, extend the database), you must make sure that
other applications can recognize the files associated with the
application. For example, you might want to add the following:
-
image/png files should be edited using the
Gimp.
-
image/png files should be described in English as
Portable Network Graphics files.
-
Files whose names end in .png should have
the type image/png.
You can use a graphical editor (such as MIME-Editor to edit the database, or you can do it manually
by creating a file called
$XDG_DATA_HOME/mime/packages/Override.xml
in the format described below. For information on the XDG_
variables, see the Base
Directory Specification.
When your new application is installed, it should install a file
with the application's name in
$XDG_DATA_DIRS/mime/packages.
For example, running the ./configure
&& make install command with the Gimp will
create
/usr/local/share/mime/packages/gimp.xml.
This file has the following format:
<?xml version=”1.0” encoding=”UTF-8”?>
<mime-info xmlns=”https://www.freedesktop.org/standards/shared-mime-info”>
<mime-type type=”image/png”>
<comment xml:lang=”en”>PNG image</comment>
<comment xml:lang=”af”>png bleed</comment>
...
<magic priority=”50”>
<match type=”string” value=”\x89PNG” offset=”0”/>
</magic>
<glob pattern=”*.png”/>
</mime-type>
</mime-info>
This provides a comment in two languages, a rule to recognize
PNG files by their contents, and a rule to recognize PNG files by
their names. You can provide information about several types in the
single
application.xml file.
You do not need to provide any information which is also in the base
package.
You can also add extra elements if they are namespaced to avoid
conflicts. For example:
<desktop:can-edit-with>gimp.desktop</desktop:can-edit-with>
This indicates that the named desktop entry file describes an
application that can edit image/png files.
Information added to the database should be static (for example,
“The Gimp can edit PNG files.”), not
configuration (for example, “The Gimp is the preferred
editor for PNG files.”). For more information on storing
configuration information, see the Shared
Configuration System Spec.
After you have installed the
application.xml
file, run the update-mime-database command to
rebuild the output files. This program checks that the syntax of your
file is correct and merges the information in it with the information
in the other XML files in the packages
directory. It then puts the rules for recognizing files into one set
of files and the information about each type into other files (for
example,
$XDG_DATA_DIR/mime/image/png.xml)
where other programs can easily access it.
When the application is uninstalled, the
application.xml
file is removed. Run update-mime-database again
to remove the information from the database.
