Just discovered something interesting; gmail won't allow you to recieve RAR files. I had the occasion to need to recieve a RAR file with some documentation in it and discovered, much to my dismay, that gmail blocked the arbitrary extension ".rar". I changed the file to .tar, which passed through unimpeded, thus showing that no file-type detection is being used.

Quite disturbing for those who use RAR for things other than piracy.