I assume you're talking about a bend too narrow for conventional needle files. I'd probably start very carefully with a fine jeweler's saw to clean out any big burrs in the crevice, then follow that with adhesive backed wet dry sand paper that's folded on itself. Folding it gives it more rigidity, still leaving it very thin, but with cutting surface on both sides. I would also use thin pieces of sheet metal to back the sand paper as far into the crease as possible.