First step would be to see if the files are truly good quality carbon tool steel throughout, or just case-hardened soft iron.
A quick way to check out the quality of the steel, and also make a usable simple flint striker at the same time and without any forging or heat-treating would be to make that old "boy scout" flint striker. The BS used to teach the kids how to make their own.
Clamp your file in the vice with around 3 inches sticking up above the vice jaws. Drape a shop rag over it. Now smack it hard with a hammer on the side right above the jaws - to snap off the part sticking up above the jaws. The rag helps control any "shrapnel" from the breaking, and also helps you find the chunk back after it flies across your shop and under a workbench. Now go to your bench grinder and grind the teeth off of the narrow edge. Cool it often - especially if it is getting too hot to hold in your fingers. You need to grind all the teeth off on that edge until you get down to full bare metal. Also bevel the teeth back a bit from that narrow edge. And grind any sharp edges/points where you broke it off. This helps save knicks/cuts.
Your piece of file should now work well as a flint striker - if it is made from good quality high carbon tool steel throughout. You will have to use the "pinch grip" to hold it. But the teeth on the flat sides will help you hold it.
Or you could cut off a small chunk from the end and the see if it will heat-treat. This is what a lot of knife makers do before they forge up a knive blade from a file. It saves them from wasting their time on a junker file.
Once you know you have a good file, then the rest is just cutting it to a more usable size, forging to shape, and properly heat-treating. I've got the steps I use posted on my web site. And there are a bunch of other web sites that have the steps to forge a striker on them as well.
Hope this helps.
Mikey - that grumpy ol' German blacksmith out in the Hinterlands
