OK, I've read up a little bit more on all of this, and thanks to
@aeric , I believe I can just send the session identifier (or authorized API key, basically) in the header, and then use PostBytes() to send the object to the server.
The data I'm wanting to send, right now, is a tiny amount. Might be between 10Kb and 50Kb for now. I will make it future-proof, in case things go wild and they need to store more. So, for now, I'd like to store the "file" directly in the MySQL database. I'm amenable.