A black English gun flint is about 7 on the Mohs hardness scale. I've read that when converted from the Rockwell C scale hardened steel roughly ranges from about 7-8 on the same scale. A good steel file would be at or near the upper end of that range, so you could theoretically work down the peak on the back of a flint with one, but like Osseon said, it would take a really, really long time. Cheap diamond (10 Mohs) files are easily obtainable these days and are a better option. If you do go with a diamond file, the most important thing I've learned in using diamond plates to sharpen my knives is to use very light pressure and let the diamonds to the work; otherwise you can tear the little tiny diamond bits out of the nickel plating they are imbedded in on the surface of the plate or file.