RestSharp is handy .NET library for doing REST requests, and it claims to support Multi-part form/file uploads. It doesn't provide any documentation on how to do this though, and I recently got tripped up trying to figure it out.

It turns out to be pretty easy though.

    //The 2nd parameter is a short-hand of (stream) => fileStream.CopyTo(stream)
    request.AddFile("fileData", fileStream.CopyTo, filename);
    request.AlwaysMultipartFormData = true;

    //Add one of these for each form boundary you need
    request.AddParameter("key", "value", ParameterType.GetOrPost);

    RestClient.Execute(request);

That's all it takes.