Ok, Martin. Let's deal with the basics. First of all we'll assume that the file was made from a steel that is good for making knives from and not low carbon surface hardened steel.
The next problem is supplying the heat. First of all it need to be applied as evenly as possible in the section that you will be wanting to heat. That can be kind of difficult with a propane torch. A oxygen/acetylene torch with a rosebud tip would be much better and a forge better still. Heating until red may or may not do the trick and ambient light can alter the preception of color. I recommend that you heat slowly in the forge until the steel will stop attracting a magnet, then get it just a little hotter (brighter) and try to hold it at that shade for about 5 minutes. The reason that I use that time is that, if the file is not case hardened mild steel, it will be supersaturated with carbon (contain more carbon than can be disolved in the hot steel) and needs a little more time to disolve it.
For quenching I'd stay away from motor oil. It sometimes has stuff in it that you don't want to put into the atmospher with hot steel also it might be a little slow cooling for the steel that you are using. I'd use a vegatable oil and only go to water if that fails. After you quench the steel you need to get it into a tempering oven right away to reduce the brittleness that will occure if the carbon is high enough. That can be the kitchen oven or you could by a toaster oven and use it, especially if the wife/girlfriend/mother objects to the former idea. Always go by a thermometer and not the dial when setting the heat. I recommend two, two hour cycles and a third cycle wouldn't hurt.
Pardon me if I'm wrong, but you sound like you don't have any books to refer to. The $50 Knife Shop by Wayne Goddard is real good for the beginner. If you are forging your blades The Complete Bladesmith by Jim Hrisoulas is probably a little better.
Doug
P.S. BTW, post pictures when you're done. We'd love to see your work.
