fix file upload in overlays
fix file upload in overlays

To include files in our upload this uses the FormData API.

Unfortunately, FormData is only available in IE 10+. In
older IEs, the whole form will be unusable.

To limit the damage this does only use FormData where necessary.

DanielNeugebauer/adhocracy,liqd/adhocracy,DanielNeugebauer/adhocracy,liqd/adhocracy,alkadis/vcv,liqd/adhocracy,phihag/adhocracy,phihag/adhocracy,liqd/adhocracy,DanielNeugebauer/adhocracy,DanielNeugebauer/adhocracy,phihag/adhocracy,alkadis/vcv,DanielNeugebauer/adhocracy,alkadis/vcv,alkadis/vcv,phihag/adhocracy,alkadis/vcv,phihag/adhocracy