Which is to say that every file has a globally 1-to-1 mapping to its encrypted version. I'm not sure how they are storing the (User, [(Filename,Key)]) data, but this is ideally encrypted on a per user basis, making any sort of per-user lookup attacks moot.
key = f(data);
upload(encrypt(data, key));
store_key(filename, key);
Which is to say that every file has a globally 1-to-1 mapping to its encrypted version. I'm not sure how they are storing the (User, [(Filename,Key)]) data, but this is ideally encrypted on a per user basis, making any sort of per-user lookup attacks moot.