You can use different keys for each file that you derive from the file itself. So if two user's store the same file they encrypt to the same data, but no one is

You can use different keys for each file that you derive from the file itself. So if two user's store the same file they encrypt to the same data, but no one is able to decrypt such data without knowing the file. On the user's account you would then store a list with keys for each file. Before this list is stored or updated on the server, it is encrypted on the client-side with the user's password, so that the server cannot retrieve this information. Of course, the security with such a scheme is somewhat lower than with "traditional" encryption, as you can find out who shares a given file. But the advantage of this is that you are still able to block particular files, if, e.g., required to do so by law.