Track disabled file inputs, re-enabled them temporarily for form submiss...

I think this is a good enhancement because during submission, I don't want visitors clicking on the file input control again and beginning a new upload. The easiest way to do that is to disable the control; however, the hidden form submission that takes place in iframe-transport will ignore the disabled file input control.

This change temporarily re-enables the appended file inputs, submits the form, then re-disables the ones that were disabled previously. See #2655.