Hi Dov,
Had a quick look into this feature on Moodle 2.6 and didn't find anything except hacks to the core code (not recommended). It looks like Canvas are ahead of the curve in this respect: http://guides.instructure.com/s/2204/m/4152/l/41295-how-do-i-restrict-online-submission-file-types
I've voted for the issue in the tracker. I suggest that everyone interested in this feature do the same: https://tracker.moodle.org/browse/MDL-39913 Perhaps for Moodle 2.8?
I hope this helps!