You simply open a connection to the filetransfer port indicated and send the transfer key. Upon receiving the key the server will then send you the designated file on that connection. That is once you opened the connection and sent the key, you will receive the data of the file that you can then save to disk or do whatever with.
Uploading works the same, but instead of the server sending you stuff, it expects you to send the file after sending the transfer key.
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