Utility does not properly handle WAIT when sending file.

In the method `send_file`, when `get_response` returns WAIT -- the packet number is increased regardless and the next packet is sent. The unit is then one packet behind and the ACK that is received from it is then never handled correctly by the utility.

Start by locating the send_file method and tracing how get_response handles WAIT and packet numbers. Reproduce the sequence where WAIT is returned, then follow the subsequent ACK handling. Done means WAIT no longer advances the packet state or causes the next packet to be sent prematurely.

Written by the indexing model from the issue text.