Do you know about File Extension M4P? .m4p is different than .mp4 or .m4a.
FairPlay Rights Management use File Extension M4P to sell their MPEG-4 files through iTunes Store without encryption.
The interest thing about File Extension M4P is you must authorizing your computer using your Apple iTunes. Just write down your username and password to download the file. And remember, just 5 computers that can be authorized with one account. Visit www.fileextensionm4p.com if you need more information about File Extension M4P.