I've been stuck on this for days. I am pretty sure that it is a bug as I have tried every possible combination of options in the upload call. Furthermore, the same code works for iOS, Android, Windows Phone 8.1, Windows Phone 10 and Windows Desktop 8.1. Windows Desktop 10 always fails with error code 2 (syntax error). I have also made tests with a minimal application that just tests the upload function (plus a small portion that creates the file to upload).