Glade files have no magic yet. This causes Nautilus to complain about the
reported file type (text/xml) being different from the guess at the file's
extension (.glade -> application/x-glade). Whether this is the right thing to do
for nautilus is questionnable, but anyway it can't be wrong to add some magic.
Here a proposal that works for me (don't know if this is the best way to do it...):
<match offset="0:256" type="string" value="<!DOCTYPE glade-interface" />
This is up to glade to register its mime types with the shared mime system, you
should file a bug against glade on bugzilla.gnome.org