Retrieve a file in binary transfer mode. *cmd* should be an appropriate
``RETR`` command: ``'RETR filename'``. The *callback* function is called for
- each block of data received, with a single string argument giving the data
+ each block of data received, with a single bytes argument giving the data
block. The optional *blocksize* argument specifies the maximum chunk size to
read on the low-level socket object created to do the actual transfer (which
will also be the largest size of the data blocks passed to *callback*). A