In Webkit there is currently no way to determine if a file drop is a folder or not.
* Chrome returns a FileList with a length of 0.
* Firefox returns a FileList with a length of 1, but always reports a file size of 0 for folders, so you can provide a workaround.
* Webkit allows the folder to be dropped, and the file size varies depending on the folder contents.
Bump. This is pretty important to get fixed.
Still dealing with this bug in 2018. I'd love to see it fixed.