Single term for "upload or download"?
I would say data transfer. This can be either upload or download.
I would recommend file transfer if you are transferring entire files, by analogy to FTP (File Transfer Protocol).
You want to distinguish what your interface does from general network communication, so data transfer is too generic.
Two possible terms: 1) bidirectional data transfer and 2) full duplex data transfer. Here's a good discussion in relation to USB 3.0.
Why not "load", with an "up" method and a "down" method?
Exchange can mean upload and at the same time download.