The freedesktop.org Shared MIME database provides a single way to store static information about MIME types and rules for determining a type.
The freedesktop.org Desktop Entry specification allows applications to announce which MIME types they support.
This specification solves the remaining issues: which application should open a file by default, how to let the user change the default application, and how to let the user add or remove associations between applications and mimetypes.
Users, system administrators, application vendors and distributions can change associations between applications and mimetypes by writing into a file called mimeapps.list.
The lookup order for this file is as follows:
|user overrides, desktop-specific (for advanced users)
|user overrides (recommended location for user configuration GUIs)
|sysadmin and ISV overrides, desktop-specific
|sysadmin and ISV overrides
|for completeness, deprecated, desktop-specific
|for compatibility, deprecated
|distribution-provided defaults, desktop-specific
In this table, $desktop is one of the names of the current desktop, lowercase (for instance, kde, gnome, xfce, etc.)
This is determined from taking the ascii-lowercase form of a component the environment variable $XDG_CURRENT_DESKTOP, which is a colon-separated list of names that the current desktop is known as.
The $desktop variable should be each of these values in turn.
All of the above files are referred to as "mimeapps.list" in the rest of this specification, for simplicity.
Note that the desktop-specific files can only be used for specifying the default application for a given type. It is not possible to add or remove associations from these files.
The algorithms for determining the list of all applications associated with a mime type and for determining the default application are (almost) completely unrelated, and so they are presented separately.
Adding and removing associations between mimetypes and applications is done with the following syntax in the mimeapps.list file:
[Added Associations] mimetype1=foo1.desktop;foo2.desktop;foo3.desktop; mimetype2=foo4.desktop; [Removed Associations] mimetype1=foo5.desktop;
[Added Associations] and [Removed Associations] groups may only appear in the non-desktop-specific files (ie: in files that are actually named "mimeapps.list").
The [Added Associations] group defines additional associations of applications with mimetypes, as if the .desktop file was listing this mimetype in the first place.
The [Removed Associations] group removes associations of applications with mimetypes, as if the .desktop file was NOT listing this mimetype in the first place.
Listing the same application for the same type in both the Added and Removed sections is invalid and may produce implementation-defined behaviour.
The order of the entries in the [Added Associations] entry for a given type should be in the "most preferred order", according to the implementation. As such, implementations should take care to preserve the order except in situations where they are explicitly intending to change it.
The adding and removal of associations only applies to desktop files in the current directory, or a later one (in precedence order). This means that additions and removals applied from the mimeapps.list file in $XDG_CONFIG_HOME, $XDG_CONFIG_DIRS and $XDG_DATA_HOME will override all existing desktop files, but that an addition or removal of an association using /usr/share/applications/mimeapps.list, for example, would be ignored if the desktop file of the named application was present in $XDG_DATA_HOME/applications. Put another way: added and removed associations can be thought of as equivalent to copying the desktop file from the next-in-precedence directory with respect to the directory containing the mimeapps.list (which may be the same directory) and tweaking its MimeType= line. If the same .desktop file appears again in a directory with higher precedence order, then this modified .desktop file with lower precedence would be ignored. If the addition or removal refers to a desktop file that doesn't exist at this precedence level, or a lower one, then the addition or removal is ignored, even if the desktop file exists in a high-precedence directory.
The suggested algorithm for listing (in preference order) the applications associated to a given mimetype is:
create an empty list for the results, and a temporary empty "blacklist"
visit each "mimeapps.list" file, in turn; a missing file is equivalent to an empty file
add to the results list any "Added Associations" in the mimeapps.list, excluding items on the blacklist
add to the blacklist any "Removed Associations" in mimeapps.list
add to the results list any .desktop file found in the same directory as the mimeapps.list which lists the given type in its MimeType= line, excluding any desktop files already in the blacklist. For directories based on XDG_CONFIG_HOME and XDG_CONFIG_DIRS, there are (by definition) no desktop files in the same directory.
add to the blacklist the names of any desktop files found in the same directory as the mimeapps.list file (which for directories based on XDG_CONFIG_HOME and XDG_CONFIG_DIRS, is none)
repeat the last four steps for each subsequent directory
The above process is repeated for each mimetype from the most specific to the least specific. Note in particular that an application "Added" with a more specific mime type will keep that association, even if it is "Removed" in a higher-precedence directory, using a less specific type.
Indicating the default application for a given mimetype is done by writing into the group [Default Applications] in the file mimeapps.list.
The [Default Applications] group indicates the default application to be used for a given mimetype. This is for instance the one that will be started when double-clicking on a file in a file manager. If the application is no longer installed, the next application in the list is attempted, and so on.
This example ensures that the application default1.desktop will be used for mimetype1, if it's installed and associated with the type, and otherwise default2.desktop if it's installed and associated:
[Default Applications] mimetype1=default1.desktop;default2.desktop;
The value is a semicolon-separated list of desktop file IDs (as defined in the desktop entry spec).
In the absence of such an entry, the next mimeapps.list is checked. Once all levels have been checked, if no entry could be found, the implementation should pick the most-preferred .desktop files associated with the mimetype, taking into account added and removed associations as per the previous section.
The suggested algorithm for determining the default application for a given mimetype is:
get the list of desktop ids for the given mimetype under the "Default Applications" group in the first mimeapps.list
for each desktop ID in the list, attempt to load the named desktop file, using the normal rules
if a valid desktop file is found, verify that it is associated with the type (as in the previous section)
if a valid association is found, we have found the default application
if after all list items are handled, we have not yet found a default application, proceed to the next mimeapps.list file in the search order and repeat
if after all files are handled, we have not yet found a default application, select the most-preferred application (according to associations) that supports the type
The above process is repeated for each mimetype from the most specific to the least specific. Note in particular that an application that can handle a more specific type will be used in preference to an application explicitly marked as the default for a less-specific type.
Note that, unlike adding and removing associations, a desktop ID set as the default for an application can refer to a desktop file of the same name found in a directory of higher precedence.
Note as well that the default application for a given type must be an application that is associated with the type. This means that implementations should either ensure that such an association exists or add one explicitly when setting an application as the default for a type.