Prefix shouldn't be interpreted as Base64 data

**warning**: this is a backwards incompatible change.

Please test it.

Also, the [spec](https://github.com/cryptocat/cryptocat/wiki/OTR-Encrypted-File-Transfer-Specification#sending-a-file) says that the prefix is encrypted with the plaintext. That's not the case in the implementation. It's mac'd with the ciphertext, which is what I think was intended.