Most (if not all) of the plate holders made for FotoKor cameras were designed to use glass plates. In theory they should be able to hold film too, but I've found that film would either fall or buckle once the dark slide is drawn when the holder is placed in the camera.
Some of the FotoKor plate holders I found came with clear glass in them. I would assume that these glass plates were used as spacers or even 'pressure plates' to bridge the thickness differences between the thicker glass plates and thinner sheet films. With glass behind the cut film, the flexible film will then be held taut between the grooves of the plate holder.
I've used the Fotokor a few times with real film (9X12 cm sheets trimmed from more common 4X5 inch cut film). In more instances however, I used ordinary bromide paper instead. The paper would have a sensitivity which approximates the typical speed of photographic plates from the times when FotoKor and other similar cameras were used. The slower speed suits the FotoKor's limited (1/25-1/100) shutter speed range. The limited colour sensitivity of photopaper also mimics the non-panchromatic emulsions of that period.
The paper negatives are then scanned to produce the negatives. Printing them directly (i.e., by 'contact printing') in the traditional way would make the resulting positive look gritty, since the paper's structure will also print through.
An example of a FotoKor portrait, shot using photopaper; "positive" made by scanning the paper negative and reversing the tones in PS:

JJ
sharp-eyed kitty