MIME Types
This chapter describes how applications detect MIME types,
how to register MIME types, and how to add applications to the GNOME Desktop.
Introduction to MIME Types
MIME types
introduction
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 email applications
use the MIME type to handle files of different types. For example, an email
application can use the MIME type to detect what type of file is in a file
attached to an email.
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.
If you add a new application, you must ensure that other applications
can recognize the files associated with the application. You must perform
several tasks to enable other applications to detect the MIME type of the
application files.
This section describes how applications detect the MIME types of files,
and how applications are associated with MIME types. This chapter also describes
the procedure that you must follow to add a new application.
Detecting the MIME Type for a File
MIME types
detecting
detecting MIME types
Applications
can detect the MIME type of a file as follows:
The application uses file content sniffers
to search for a particular pattern in the file. A file content sniffer associates
a specific pattern in a file with a MIME type. If the application finds a
match for the pattern, the MIME type associated with the pattern is the MIME
type of the file.
If file content sniffers do not identify the MIME type, then
the application can check the filename. The application checks the filename
against the MIME type registry. The MIME type registry
associates particular file extensions and filename patterns, with particular
MIME types. If a match for the filename is found, the MIME type associated
with the extension or pattern is the MIME type of the file.
The following sections provide further information on file content sniffers
and the MIME type registry.
File Content Sniffers
MIME types
file content sniffers
file content sniffers
File content sniffers are specified in the file /etc/gnome-vfs-mime-magic. The following is an example of a file content sniffer:
0 string \x89PNG image/png
The syntax for file content sniffers is as follows:
offset_start[:offset_end] pattern_type pattern [&pattern_mask] type
describes the fields in a file content
sniffer.
Fields in a File Content Sniffer
Field
Description
offset_start
Specifies the number of characters to ignore in
the file before searching for a text pattern.
pattern_type
Specifies the type of pattern to search for. The string pattern type is the only pattern type that is supported at
the time of publication of this guide.
pattern
Specifies the pattern to search for.
pattern_mask
Specifies a pattern mask,
in hexadecimal format. For more information on pattern masks, see the next
section.
This field is optional. This field is not present in
the example.
type
Specifies the MIME type to associate with files that match
this entry.
Pattern Masks
MIME types
pattern mask
pattern mask
A
pattern mask identifies bits in the pattern to ignore when searching for a
pattern in a file. The following is an example of a file content sniffer with
a pattern mask:
0 string BMxxxx\000\000 &0xffff00000000ffff image/bmp
The pattern and mask in the example are as follows:
Pattern
B
M
x
x
x
x
\000
\000
Mask
ff
ff
00
00
00
00
ff
ff
The pattern and mask specify a file with the following characteristics:
The file begins with BM.
BM is followed by four bytes with any values.
The four bytes are followed by \000\000.
The file content sniffer specifies that the MIME type of files that
match the pattern and mask is image/bmp.
MIME Type Registry
MIME types
MIME type registry
The MIME type registry is located in /usr/share/mime-info. The MIME type registry contains the following files:
File
File Extension
MIME information file
.mime
MIME keys file
.keys
The following sections describe MIME information files and MIME keys
files.
MIME Information Files
MIME types
MIME information files
MIME information files
associate MIME types with one or both of the following:
File extensions
Filename patterns
When an application searches for the MIME type of a file, the application
checks the filename against the MIME information files. If a match for the
filename is found, the MIME type associated with the extension or pattern
is the MIME type of the file.
In MIME information files, the filename pattern to search for is written
as a regular expression.
The format of MIME type entries in MIME information files is as follows:
MIME-type
ext[,priority]: list-of-extensions
regex[,priority]: list-of-regular-expressions
You can specify a priority value for the file extension and the regular
expression. You can use the priority value to differentiate composite filenames.
For example, you can assign a priority of 1 to the .gz extension, and assign a higher priority of 2
to the .tar.gz extension. In this case, the file abc.tar.gz takes the MIME type for .tar.gz.
You must indent the ext field and the regex field with a tab character (\t).
The following MIME type entries are samples from the gnome-vfs.mime MIME information file:
application/x-compressed-tar
regex,2: tar\.gz$
ext: tgz
audio/x-real-audio
ext: rm ra ram
image/jpeg
ext: jpe jpeg jpg
image/png
ext: png
text/html
ext: html htm HTML
text/plain
ext: asc txt TXT
text/x-readme
regex: README.*
The file manager reads the MIME information files alphabetically.
The alphabetical order determines the order in which MIME types are assigned
to file extensions or regular expressions. For example, if the same file extension
is assigned to different MIME types in the files abc.mime
and def.mime, the MIME type in abc.mime
is used.
MIME Keys Files
MIME types
MIME keys files
MIME keys file provide information
about a MIME type that is used in the user interface. For example, the MIME
keys file provides a description of a MIME type, and specifies an icon to
represent files of that MIME type.
The following is a sample from a MIME keys file:
text/html
description=HTML page
icon_filename=gnome-text-html
default_action_type=application
short_list_application_ids_for_novice_user_level=mozilla,netscape,galeon
category=Documents/World Wide Web
You must indent the keys in a MIME keys file with a tab character
(\t).
describes the most important keys in
MIME keys files. Typically, the description key and the category key are localized.
Keys in MIME Keys Files
Key
Description
can_be_executable
Specifies
whether files of this MIME type can be executed.
description
Describes the MIME type. This description can be
displayed in the file manager and other applications.
icon_filename
Specifies the filename of an icon to represent the
MIME type. Does not specify the path to the filename, or the file extension.
This icon can be displayed in the file manager and other applications.
default_action_type
Specifies the category of action to take when a
file of this MIME type is opened by the user. Enter application
for this MIME type for most applications.
short_list_application_ids
_for_novice_user_level
Specifies the application to use when a file of this MIME type is opened by
a user. Specify one or more applications, in order of priority. The applications
must also be registered in the application registry.
category
Specifies a category for the MIME type. The value
of this key determines the location of the MIME type in the File Types and Programs preference tool.
Registering Applications for MIME Types
MIME types
registering applications
for
applications
registry
The application registry contains text files that register applications. The application
registration files contain a series of key-value pairs that specify details
for applications. For example, the application registration files contain
the following information:
The command to use to start the application.
MIME types to associate with the application.
An application registration file can contain one or more application
registrations. Application registration files have a .applications extension.
The location of the application registry is /usr/share/application-registry. This directory contains a default application registration file
that is called gnome-vfs.applications.
To register an application, add a registration file for the application
to the application registry.
The following is an example of an application registration:
eog
command=eog
name=Eye of Gnome
can_open_multiple_files=true
expects_uris=false
requires_terminal=false
mime_types=image/bmp,image/gif,image/jpeg,image/png,image/tiff,
image/x-xpixmap,image/x-bmp,image/x-png,image/x-portable-anymap,
image/x-portable-bitmap,image/x-portable-graymap,
image/x-portable-pixmap
describes the keys in application registration
files.
Keys for an Application Registration
Key
Description
Application identifier
Specifies a unique identifier for the application. This
identifier must be the same as the identifier in the short_list_application_ids_for_novice_user_level
key in the MIME keys file for the application.
command
Specifies the command to use to start the application,
and any options to use with the command.
name
Specifies a name for the application. The name is used
in the user interface. For example, the name is used in the Open
With submenu in the file manager.
can_open_multiple_files
Specifies whether the application can open several
files at the same time.
expects_uris
Specifies whether the application can process URIs.
If the value of this key is true, the application registration
entry must also contain a supported_uri_schemes key.
supported_uri_schemes
Specifies
the URI schemes that the application can process.
requires_terminal
Specifies whether to run the application in a terminal
window. Enter true for this field for an application that
does not create a window in which to run.
mime_types
Specifies the MIME types that the application can
use.
Adding an Application to the GNOME Desktop
MIME types
adding applications
applications
adding
To add an application to the GNOME Desktop, perform the
following steps:
Add a menu item for the application. For more information
on how to add an item to a menu, see .
Add an icon for the application to /usr/share/icons/theme-name/icon-size/apps.
For more information on icons and themes, see .
If the application uses a new MIME type, add a file content
sniffer for the new MIME type. For more information on file content sniffers,
see .
If the application uses a new MIME type, add a MIME information
file for the application to the MIME type registry. For more information on
MIME information files, see .
Add a MIME keys file for the application to the MIME type
registry. For more information on MIME keys files, see .
If the application uses a new MIME type, add an icon for the
MIME type to /usr/share/icons/theme-name/icon-size/mimetypes. For more information on icons
and themes, see .
To associate the application with a MIME type, add an application
registration file to the application registry. For more information on the
application registry, see .