Get a utility knife and chip/scrape the paint off the insides of the fork ends. Files can be dangerous in the hands of someone who is not used to using them (ie, no control over material removed). Sandpaper/sanding sticks (available at better hobby shops) are also a possibility. You don't want to remove metal, if you can avoid it, just paint.

That said, everyone else is on the right track. Avoid swapping parts until you try removing the painted surface.