Consult: Transfer files using Vysper and Smack.

[aazc]

Hi there,
I'm now developing an XMPP program. I'm trying Vysper, while I failed to 
transfer files between clients.
Will the file be uploaded to server and send to the other client? Or the 
transfer traffic is only between clients?
I'm using the full story Vysper, Smack as the client, and trying to transfer 
files following it's official guide. The smack API just shows that the transfer 
progress is 0. Did I miss anything on the server side?