Here's a picture of what it should look like.
Note the torque arm, and the side flatted axle. Without a torque arm the axle will try to rotate and act like a cam forcing the dropout open with tremendous force. With a torque arm, the axle is stabilized and doesn't unduly stress the fork
The sharp edge you left probably didn't help, but wasn't the main issue. If you had a torque arm on, I doubt the file work would have made much difference.