You are right, there is no MITM protection on the first connect, only on following connects. Adding a PKI would probably be overdoing it. We use the same encryption (AES-256-EAX) for the text chat like we use for voice chat, except that it is not optional.
There are internal plans for adding encryption to the filetransfer, but these will probably be part of a mayor overhaul of the filetransfer that will not be backward compatible. There is currently no time tables for these changes.
