Don't file either!!! You can spread the rear-triangle outwards. Shimano makes some axle lock-nuts that have a beveled outer edge which helps to spread out the rear-triangle as you push the wheel on.
If it's the slot in the drop-outs that's too narrow to accept a 10mm axle, then you can use a big screwdriver slid down the slot (parallel to plane of dropout) and pry apart the slot slightly.