This file type is designated for mp4s, or videos, that can legally be screenshotted and turned into transparent images. If you are just viewing a regular mp4 file, and you screenshot a part of it and (using some photo editing software) make the background transparent, you can end in a lot of trouble. It is still done, but it is heavily frowned upon by the government.
This file type was created for banks like Barclays to virtually transfer money between accounts, since the old fashined way of shipping money abroad to store in offshore vaults was too much of a hassle for private investors. Unfortunately, all was going well with this file type, until maleovalent hackers made their way into many banks' central security systems and took all the money files for there own. You can store up to €59,131,245.00 on one .money file, so the hackers, who were never discovered, and balling in riches (needless to say).