Chapter 5 MIME Types

This chapter describes MIME types, and the MIME database, and also describes how to add applications to the desktop.

Introduction to 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 web 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 attached to an email.

The 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 to the desktop, you must ensure that other applications can recognize the files associated with the application. You must ensure that if the new application requires a new MIME type, that the new application installs the new MIME type.

This chapter describes the MIME database, and how applications are associated with MIME types. This chapter also describes the steps that you must follow to add a new application to the desktop.

MIME Database

All MIME type information is stored in a database. The MIME database is located in the directory /usr/share/mime/. The MIME database contains a large number of common MIME types, stored in the file /usr/share/mime/packages/freedesktop.org.xml. Applications can add new MIME types to the MIME database.

Applications use the MIME database to detect the MIME type of a file, in the following ways:

• The application checks the contents of the file, then uses the MIME database to identify the appropriate MIME type. The MIME database contains file content sniffer information. The file content sniffer information provides details of a specific pattern in the file. The MIME database associates this pattern with a MIME type. The application checks the file for the pattern. If the application finds a match for the pattern in the file content, the MIME type associated with the pattern is the MIME type of the file.

  The magic elements in the MIME database specify the file content sniffer information.

• The application checks the filename, then uses the MIME database to identify the appropriate MIME type. The MIME database associates particular file extensions and filename patterns, with particular MIME types. The application checks the filename to search for a particular pattern in the filename. 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 glob elements in the MIME database specify the filename pattern and file extension pattern information.

The MIME database also contains a text description of each MIME type. The MIME database can also contain a list of applications that you can use to view or edit a MIME type.

Structure of the MIME Database

The following is a sample from the MIME type database:

<mime-type type="image/png">
		<comment>PNG image</comment>
		<comment xml:lang="de">PNG-Grafik</comment>
		:
		:
		:
		<magic priority="50">
			<match offset="0" type="string" value="\x89PNG" />
		</magic>
		<glob pattern="*.png" />
	</mime-type>

Table 5–1 describes the elements in the MIME type database.

http://www.freedesktop.org/standards/shared-mime-info

Attributes in Match Elements

Table 5–2 describes the attributes in a match element.

Files Generated From the MIME Database

When a new MIME type is installed, the update-mime-database command is executed to update the MIME database. Table 5–3 describes the files that the update-mime-database command generates:

application/x-bzip-compressed-tar:*.tar.bz2
text/html:*.html
image/png:*.png

Adding a MIME Type to the MIME Database

To add a new MIME type to the database, an application must install a MIME type description file. A MIME type description file is an XML file that associates MIME types with file content sniffer information, and with file extensions and filename patterns.

The following is a sample MIME type description file:

<?xml version="1.0" encoding="UTF-8"?>
<mime-info xmlns='http://www.freedesktop.org/standards/shared-mime-info'>
	<mime-type type="image/png">
		<comment>PNG image</comment>
		<comment xml:lang="de">PNG-Grafik</comment>
		:
		:
		:
		<magic priority="50>
			<match offset="0" type="string" value="\x89PNG" />
		</magic>
		<glob pattern="*.png" />
	</mime-type>
</mime-info>

The elements in a MIME type description file are similar to the elements in the MIME database. For information about the elements in a MIME type description file, see Structure of the MIME Database.

After the application installs the MIME type description file, the application must execute the update-mime-database command to update the MIME database.

Adding an Application to the Desktop

To add an application to the desktop, perform the following steps:

1. Add a menu item for the application. For more information about how to add an item to a menu, see Chapter 2, Customizing Menus.

2. Add an icon for the application to /usr/share/icons/theme-name/icon-size/apps. For more information about icons and themes, see Chapter 3, Installing Themes.

3. If the application uses a new MIME type and does not automatically add the MIME type to the MIME database, add a MIME type description file to /usr/share/mime/packages/.

4. If the application installs a new MIME type description file, ensure that the update-mime-database command is executed during the installation, to update the MIME database.

5. If the application uses a new MIME type, add icons for the MIME type to /usr/share/icons/theme-name/icon-size/mimetypes. For more information about icons and themes, see Chapter 3, Installing Themes.

6. Log out of the session, then log in again.