Grind it. I don't think files are going to do anything for it.
Draw the new shape you want on it with a Sharpie pen then put a large container of water next to your grinder & grind it to shape s l o w l y!
Don't try to remove too much metal in each pass.
After each pass or at most every other, dip the blade in the water to cool it. When it comes to grinding tempered steel slow is good.
If the metal is too hot to hold in your bare fingers, you're going to risk the temper in it!!
Take your time w/ the grinding & finish it up with finer & finer wet/dry paper down to about 600 grit.
Larry