A stiff wire brush and some vinegar will help, don't fall for the tale of restoring files in an acidic solution, it doesn't work, get yourself a file card and definitely place a very very light coat of oil on them where you're done. Once a files done, it's gone for good in my experience
I too was a non-believer, but I have to say this is DEAD WRONG!
It not only works, but works well. I have more files than any sane person would likely own up to. I am also too cheap to easily turn loose of a dull file, but finally bit the bullet a couple of years ago and sorted the good from the dull - with the intention of pitching the dull ones into the scrap metal bin. Having nothing to lose, I decided to put what I had been reading to the test and proceeded to soak all the dull files in high-strength cleaning vinegar. To say I was favorably impressed would be an under statement. Nearly all of them were sharpened to at least nearly new condition. The two or three that were still not worth keeping had damage that exceeded just being dull.
Here is a good example of the results:
I have an old 12" lathe file that was so dull that I considered it unusable and it was one of the files I soaked. I also have a brand new Mexican Nicholson 12" lathe file. I know - not the best quality. A well know local builder was visiting in my shop just yesterday and, by coincidence, he brought up the subject of sharpening files. After explaining the process, I chucked a piece of mild steel in a vise and handed him the two lathe files. He was surprised to discover the sharpened file cut noticeably better than the new one.
I was once in denial but am now a believer
On the subject of file cards:
I no longer drag hardened steel pins across the teeth of my sharp files. I cut pieces of plastic laminate counter top (Arborite, Formica, etc.) into roughly 1 1/4" X 4" strips and use them to clean my files. Just push the narrow end across the teeth. It will just take a few passes to cut the tooth pattern into the laminate. The laminate is very effective at cleaning the gullets and will not dull the teeth.
OK, I am going to get off my soap box now.