And it doesn't have to be a file. I installed a couple of those while on a visit with some friends, neither of whom had a file. But one of them did have one of those flat diamond grit-coated metal knife sharpening plates. It took a bit of rubbing because the side cuts leave extra metal and need to be reduced some to fit width of the recess in the slide and the extractor retainer groove, but in a little while I had a good fit. The same tool produced a perfectly serviceable light radius.
By the way, these really do work well. The ones I put in were as part of a trip to do a lot of shooting on plates and dueling trees and the like. We could all see there was a lot less vertical disturbance during recoil, so the sight picture returns fast.