A way to resharpen old files
According to Henley's Twentieth Century Home & Workshop Formulas Recipes & Processes (1927): "Old files may be rendered useful again by the following process: Boil them in a potash bath, brush them with a hard brush and wipe off. Plunge for half a minute into nitric acid, and pass over a cloth stretched tightly on a piece of wood. The effect will be that the acid remains in the grooves, and will take away the steel without attacking the top, which has been wiped dry. The operation may be repeated according to the depth to be obtained. Before using the files thus treated they should be rinsed in water and dried."
Be careful handling acids.
The Guild of Metalsmiths has not tested these techniques, and is offering them as is. Anyone using them uses them at their own risk.