FileTransfer.upload() documentation

I am attemting to call the upload url with FileTransfer.upload()

I am supplying a "files" and "postvars" map. In some situations, I have content in the "postvars" map, but not in the "files" map. When that situation occurs, BrowserPlus is NOT calling the url with the "postvars", thus disrupting my application.

It is documented that the "files" argument is required, but I still don't know whether the absence of content in the "files" map is the reason for not calling the url with the "postvars". Can you please shed some light on the requirements for that method?

Thanks.