File transfer in Telepathy is handled using a
Channel.Type.FileTransfer channel.
The connection manager carries out file transfer via the most appropriate
mechanism, depending on the protocol. Connection Managers do not know
about accessing files or URLs, and it is not possible to pass file
descriptors across D-Bus, so transfer between the client and the
connection manager is done using a local socket (e.g. a UNIX socket or
local IPv4 socket).
The lifetime of a file transfer channel goes through several states:
Pending, Accepted, Open and Completed (or Cancelled). This is shown in
Figure 9-1.
