support <proxy /> extention from Psi for file transfert.
Psi added some extensions to JEP-0096: file transfert to take into account some situations not mentionned in the JEP. Their extensions are documented on this page : http://delta.affinix.com/specs/stream.html
Their implementation works fine with clients not supporting these extensions so there is nothing to fear supporting it.
Also worth reading about file transfert is this topic on their forum : http://psi.affinix.com/forums/index.php?act=ST&f=2&t=3064&s=5d4f93fd9b8b3b0add931a78642454b9
The least that should be done would be to send proxies addresses with their <proxy /> element as it does not require a lot of changes in Gajim code. Supporting the whole extension set would be great.