Ah. I think what you are trying to do is anneal the file, to soften it, yes?
If so, you won't get any joy from a kitchen oven. To anneal, you first need to heat the file to it's critical temperature (if you get it an even 'orange' hot, you'll not be too far off the mark) You'll need to do this in an actual fire. Either a forge or a charcoal BBQ will get it there. You then need to let it cool, very very slowly. Your best bet is to throw it onto a raging hot BBQ, cover it in charcoal & let it sit there burning overnight.
That 'should' get it soft enough to work with.