Sending binary data to HTTP server
You should forget about MIME, that is only needed if you are sending multiple files to the server. Your best bet is to just send these headers, then a blank line, then your data:

POST /api/users HTTP/1.1
Accept: */*
Content-Type: application/octet-stream
Content-Length: 123


On the other end, you just look at "request.body" and it should have your binary data. (Replace 123 with sizeof(buffer))