Hi, I have this proyect i need to do, and I was hoping someone could help me put. I need to prevent users in a Linux server from writing illegal files in their accounts(mp3,pictures,mov,wav,bin,etc) wel, what I really need to know is to identify this files in case users change their extension to something like .doc,.txt,.trickyfile, etc. The specifications and restrictions are these:

•That it is usable on any Unix system (i.e. Linux,Solaris workstations & server etc).
•That it does not have a cost.
•That it is legal (no pirated commercial solutions allowed).
•That can identify the most different kinds of files possible.
•That it finishes its task in the least amount of time.

I hope someone can tell me go to do this, it will be a challenge, but I hope you guys can help me. Thanks.

NOTE: I tryed using the file command, but it doesn't detect all files....