One option is to create a little script for the phantomjs browser, either via selenium or another binding. The script could log in for you and upload whatever file you want. See the tutorials for specifics or ask here again if you get stuck. There are bindings for many programming languages, if you decide to use ghostdirver (webdriver version of phantomjs).
P.S. i hope you strongly encrypt your backups before uploading them into the cloud!