We used to use them on aircraft aluminium panels. They used to slice your fingers when your hand continued pushing, but the file stopped moving. We also used Millenicut files. These has straight teeth in sets of 5 , 4 teeth were normal and the 5th was serrated.
Dreadnaughts were a very coarse file, followed by bastards, straight cut and second cut. File cards were used to remove swarf from the file and chalk rubbed onto the file to a) stop swarf sticking to the file and b) make the cut finer for finishing.
Re-cut files had been acid dipped to "sharpen" used files.