They could still do something. In cases where they have reason to suspect this is going on (which could be based on file usage patterns), they could start aski

They could still do something. In cases where they have reason to suspect this is going on (which could be based on file usage patterns), they could start asking clients to send the file. Clients that start an upload and then disconnect when asked for the file are pretty much caught red handed, and you could just block their accounts until they upgrade to the next version.