I am a (cheap) skeptic for pretty much everything snake oil related, but deOxit works quite well for cleaning up corroded contacts. My guess is that it's some form of acid and oil mix. The stuff I have is in an aerosol can with a metered spray tip, one puff at a time. It does wonders on contacts that have been corroded by old alkaline cells that have spewed. As mentioned, it's not going to re-nickel plate anything, but when applied to heavy corrosion it dissolves things like nothing else (except acids!).......and no I don't work for DeOxit...
Filing contacts usually ends up shortening their lives but I've filed many mechanical relay contacts with good results over the years.