Blocking attachments - Stopping sneaky employees

[Jon Bates]

I've got all audio and video type files being quarantined on my servers.
Some users are now getting smart to the fact that they can simply change the
extention on the file to bypass this system.
 
Is there some way to filter attachments based on the attachment mime type or
something? I've done a few hours searching and I havent come up with a
suitable answer. 
 
Any guidance would be appreciated!
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