You can download binary files, but you can't upload them.
this is kinda going on a tangent, but since the question here has already been answered...
why not? I guess this is really a two-part question: why aren't there facilities for it in the game already? and two, what's stopping you from working around that?
like, why not just send it as regular form data (assuming you can make POST requests. if not, that raises even more questions)? you may run into trouble with larger files, but I'm sure that something small, for instance, a 300x300 save thumbnail, would be fine