If you look at the end, it is a long slender triangle. It is sorta like one half of a feather file. The knife file can be found in lengths of 4" to 8". A good place to find them is MSC Industrial supply. Google them to find them, if you are not familiar with them. They have many useful things for knifemakers, from torx screws to glass beads and metal lathes to bridgeport mills.
The feather file and the knife file are useful in fileworking because they bite into the steel very easily being such a sharp angle (perhaps 15 degrees or so). And when you wear out your knife file, it is very easy to make it into a knife! I've took a 4" one once and drew it in the oven in the kitchen until it got to 58 rc and sharpened it and handled it. I used it around the shop for 3 or 4 years. It made a nice knife.