xdg-utils

xdg-utils provides the official utilities for managing MIME types and default applications according to the #XDG standard. Most importantly, it provides /usr/bin/xdg-open which many applications use to open a file with its default application. It is desktop-environment-independent in the sense that it attempts to use each environment's native default application tool and provides its own mechanism if no known environment is detected. Examples:

# determine a file's MIME type
$ xdg-mime query filetype photo.jpeg
image/jpeg

# determine the default application for a MIME type
$ xdg-mime query default image/jpeg
gimp.desktop

# change the default application for a MIME type
$ xdg-mime default feh.desktop image/jpeg

# open a file with its default application
$ xdg-open photo.jpeg

# shortcut to open all web MIME types with a single application
$ xdg-settings set default-web-browser firefox.desktop

# shortcut for setting the default application for a URL scheme
$ xdg-settings set default-url-scheme-handler irc xchat.desktop