File upload fields disappear when you go back to edit a webform in the form builder interface

Since #1025706: Missing red asterisk was committed to Webform, the file component integration in Form Builder no longer works correctly. You can create webforms that have file components, but if you go back and edit them in the form builder interface, the file components disappear and are destroyed completely if you save it.

This is because the form builder integration code was relying on some custom webform element properties that are no longer there after that patch was committed.

The attached patch fixes the file component integration so it works again, and also cleans it up so it's a bit simpler.