XEP-0096: SI File Transfer
"The traditional mechanism for transferring files in the Jabber community is the Out-of-Band Data  protocol. That protocol has several drawbacks:
It is not reliable.
It does not work when one of the parties is behind a firewall.
It provides limited metadata about files to be exchanged.
The current document defines a profile of Stream Initiation  that solves the problems with out-of-band data, thus providing a robust, reliable mechanism for file transfers over the Jabber network. Implementors are referred to XEP-0095 regarding the underlying concepts of stream initiation."
Now what is needed is the actual file transfer extension...