Andreas Hruska, Technical University Vienna, presented a filter like this at MoodleMoot Bamberg and uploaded the other day a patch at Moodle Tracker:
http://tracker.moodle.org/browse/MDL-18713
This will not technically check the file itself but look for format name - which should do.
You'll have to ask your Moodle admin (or be it
Hope this fits you,
Andreas