Here is a patch which adds three new public functions,

int         xdg_mime_mime_type_equal              (const char *mime_a,
						   const char *mime_b);
int         xdg_mime_media_type_equal             (const char *mime_a,
						   const char *mime_b);
int         xdg_mime_mime_type_subclass           (const char *mime_a,
						   const char *mime_b);

The first one uses the alias information in the mime database to determine 
whether two mime types should be considered the same, the last one uses 
subclass and alias information to determine whether a mime type should be
considered subclass of another one. The second function compares the media
parts of two mime types for equality, and can be used when implementing 
support for "super types" like "text/*".