I've followed and managed to successfully implement this:
As I'm including the relevant fields and evaluating them like string(@HTML_x005F_x0020_File_x005F_x0020_Type)
, for example, it's creating them as slots and exposing these on the 'Edit Web Part' panel - but they are not to be displayed there - they have no purpose there. Can anybody please guide me in the right direction of hiding them from the UI?