As Steve Weis pointed out in an earlier thread about schemes like this, deriving keys from the contents of files breaks semantic security. Lay engineers reason about this problem the way you just did: "the RIAA can tell I have Lady Gaga MP3s". But practitioners are worried about much more subtle and devastating flaws, particularly in cases where attackers may exercise some control over the blocks being encrypted.

Any scheme that derives passwords from file contents gives me the willies.