4.2. Detecting the MIME Type for a File

4.2. Detecting the MIME Type for a File
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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 filename 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.

4.2.1. File Content Sniffers

File content sniffers are specified in the file /usr/gnome/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

Table 4.1 describes the fields in a file content sniffer.

Table 4.1. 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.

4.2.1.1. Pattern Masks

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.

4.2.2. MIME Type Registry

The MIME type registry is located in /usr/gnome/share/mime-info. The MIME type registry contains the following files:

File	Filename Extension
MIME information file	`.mime`
MIME keys file	`.keys`

The following sections describe MIME information files and MIME keys files.

4.2.2.1. MIME Information Files

MIME information files associate MIME types with one or both of the following:

Filename 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 filename 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.

	Note
	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.*

	Note
	The file manager reads the MIME information files alphabetically. The alphabetical order determines the order in which MIME types are assigned to filename 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.

4.2.2.2. 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

	Note
	You must indent the keys in a MIME keys file with a tab character (`\t`).

Table 4.2 describes the most important keys in MIME keys files. Typically, the description key and the category key are localized.

Table 4.2. 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 filename 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.

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