Provide the callback with relative path of files when uploading folders

I'm writing an app where I need to be able to upload folders and reconstruct the folder hierarchy in my app. I added support for this by adding a "relativePath" parameter to the callback POST.

Note, while these are 3 commits, the third is reverting the first one so it is only the middle commit that contains the actual change.

P.S. I've only tested this on Chrome.