Fix mandatory file uploads in order conditions not working

[fritzmg]

Currently mandatory file uploads in order conditions do not work. When the form is submitted, Isotope will stay at the checkout/review step, without showing any error. The problem is that when checkout/process processes the order conditions checkout step again, $_FILES will be empty at this point and thus the file upload widget will not validate and therefore the whole order condition step will not validate, even though it was valid during the POST request.

To fix this $_FILES will need to be repopulated for the cloned widget when the checkout is processed.