Files are transferred as blob. Encryption can be dropped in if required. :)
Encryption ought to be standard. Making it optional means most people won't use it.
Per spec DataChannels should use encryption by default, you must still encrypt all WebSocket or other traffic that goes via your server.
You are true. DataChannels are encripted by spec, and WebSockets are also being cyphered. I've taken security and anonimity as far as I could (I'm not a security nor frontend guy... :-P )
Encryption without authentication is (mostly) useless, and authentication is application-specific, so mandating encryption would have little effect.