FTP.transfercmd(cmd, rest=None)
Initiate a transfer over the data connection. If the transfer is active, send an EPRT or PORT command and the transfer command specified by cmd, and accept the connection. If the server is passive, send an EPSV or PASV command, connect to it, and start the transfer command. Either way, return the socket for the connection.
If optional rest is given, a REST command is sent to the server, passing rest as an argument. rest is usually a byte offset into the reques