Upload new versions shouldn't allow multiple files to be selected (e.g. by dragging and dropping a folder)

Also, it probably should not be allowed to drag and drop a folder on for a revision upload
I'm not sure there's an actual bug in the plugin; it's more like there's a side effect that we have to take into account. If we leave the `singleFileUploads` to the default (`true`), then the plugin calls the `add` callback (in our case `checkValidFile`) separately for each file in a folder, which prevents our code from detecting a folder.

That behavior is really separate from the issue of whether or not the plugin uploads multiple files in a single XHR. (In the case of the upload new version, since we're only allowing a single file, there's no benefit to allowing the plugin to combine multiple files into one XHR.)

Does that make sense?