Store a file in binary mode. A new port is created for you.
Args:
cmd: A STOR command.
fp: A file-like object with a read(num_bytes) method.
blocksize: The maximum data size to read from fp and send over
the connection at once. [default: 8192]
callback: An optional single parameter callable that is called on
each block of data after it is sent. [default: None]
rest: Passed to transfercmd(). [default: None]
Returns:
The response code.