Best alternative to a file would be a strip of that wet-or-dry 600 grit paper. Cut a strip and fold it over a thinned down popsicle stick or something like that to resemble a file. I don't use nail files, I bite my fingernails, but I suspect it may be too coarse anyhow. Or just fold the paper over on itself and push-pull it between the contacts. You only have to burnish the contacts which means you don't have to take much metal off. Just clean them up a bit.
Al