Mounting large mirror on drywall/studs with unknown brackets
If there are brackets (straps) on both sides about 1/3 of the way down from the top, then you could use S-hooks in the straps (remove the center clamping screw) and string a heavy steel wire between them. Then hook the wire on a heavy nail into a stud or use one of the new stronger hangers like a WingIt into drywall.