Process FileField inside Multiple groups

Two bugs were preventing FileField elements from being correctly processed when inside a Multiple - the first was a spelling mistake in Form which meant that setValues was not being properly called in Multiple and the other was that, due to the way files are posted, some processing was needed on the array to get it into a format that the rest of Formidable can understand
Apologies, I've cherry-picked this from some other changes - do you want me to delete this pull request and open a new one?