Files are a deceptively simple invention. Using the basic idea of friction and abrasion, one can render dangerously sharp corners and edges into a smooth, rounded corners and edges.
I wonder what mankind did before the invention of files, especially to keep nails in check. Grab a ‘scratching post’ like a log? (Just thinking about all the splinters makes me cringe, though.) Bite and nibble them short, like some of us still do? I’m betting on the latter. Take some time to thank your nail file when you see it.