If the file is a Nicholson and stamped USA or canada, it's W2, and will make a very good knife. The only thing I will add is that unless you have a full size belt grinder (2x72) you will probably spend more on abrasives removing the teeth, then you would to order a nice new, surface ground piece of starrett O1 in a nice paper wraper from kbc tools. I started out only making file knives, and still make a few, but the more knives I make, the more I just buy new steel. So much faster and easier, and if you factor in abrasives (let alone time) it's cheaper.
And if you ever do get into knifemaking and decide to try a knife in stainless or some fancy steel, I'll offer to heat treat it.
One trick to tell if steel is any good for blades is to grind it. Take a piece of normal mild steel and grind it. Look at the sparks. Then grind a file. You'll notice that the sparks from the file are a lot more feathery. This indicates a higher carbon content. The higher the carbon, the more noticeable the feathers.
you can never have too many tools