Once again, stick with Nicholson and Simonds files. There may be other brands that are suitable, but I don't know that for sure.
Kitchen ovens are notoriously inaccurate and often fluctuate quite a bit, so start on the low end the first time you re-temper a file. You may want to start at 350 and do another temper at a higher setting if you don't get a nice pale yellow/up to bronze oxide color on the steel. An hour "soak" once the oven is warmed up is probably enough to get the temp even throughout a thin piece like a file/blade. Leaving it in there longer won't hurt.
Gary is 100% right about cooling often when working with any hardened steel. Keep your fingertips on the file/blade as you grind and dip in water as soon as it starts to feel hot. This will prevent you from overheating the steel and drawing back the temper too much.
Even though you cool the file often, you may find that it holds a fair amount of heat after several passes on the grinder, enough to be uncomfortable against your fingers. It never hurts to take a break and let it cool to room temp.
If the edge turns blue from too much friction/heat, it will be quite soft and won't hold a keen edge worth a darn. Your only option is to anneal it and re-heat treat... which pretty much strips away the whole point of using a good file to make a knife. (ie, it's already HT'ed and if you temper it back a bit as described above, it's grindable and will still take and hold a good sharp edge)
It's possible to shape a re-tempered file with hand tools (files and sandpaper), but I wouldn't advise it unless you're extremely bored and have a lot of time to kill.