I use mill files on wood, plastic laminate, veneers and metal laminates. File cards and stiff brushes are the common methods for cleaning files but in no way will they sharpen them.
After I had a stash of a coupla dozen I had a brainstorm of cleaning them in a bath of muriatic acid (just the ordinary pool type). I used a plastic tray with sides, and set the files in on edge and let 'em soak. Well, when I finally pulled them out, they were clean alright, and were sharper.
I must say that this is an incredibly hazardous method, as the acids emits toxic fumes and more so when the acid was added to the files. If tried it should be done outside, away from the house or shop. Appropriate gloves, arm and face protection should be used. Proper disposal of the used bath should taken as with any hazardous material.
It won't make them like new, but will give some new life.
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Your almost extinct silversmith will tell you that this is a task that you farm out the experts who do this kind of thing. Result comes back pretty much a sharp as day one sealed in a plastic wrap. Use to do this as soon as I found any build up I couldn't brush away. Trick is to have enough overlap so that you don't have to stop work while you wait. Find a local jeweler/smith who actually still makes things and inquire about who s/he uses for this...
p.s. you aren't using the same file on more than one kind of material like wood and metal are you? If so, don't.