lib/classes/filetypes.php defines the filetypes included in core.
get_default_types returns an associative array of extension keys to a value which is an array of type, icon, groups and string (which is a lang string which will be used when displaying the type. The lang string is passed $a->EXT which is the actual extension.
Any groups added should add a language string to lang/en/mimetypes.php with the formate group:<groupname>. This is so that the group can be displayed in a human readable formate in the user's native language.