I would like to be able to search by the extension of a particular file.

If I am looking for a movie, I would like to be shown results that are mpg or mpeg, not avi, mov, rm and what have you.

This will also be useful in distinguishing better between Mac and Windows applications. If I was a windows user, I would be mostly searching for .exe, but as a Mac, I would be more interested in .dmg or .sit.