In use, I have personally found that I get better sparks when I have a narrow or thin edge on my flint striker to strike sparks with. What you are trying to do is use a sharp edge on your flint to chip/dig little bits of steel from the striking surface or your striker. The energy you put into chipping/digging out those little bits heats them up enough that the carbon in those little bits burns - that's the sparks you see.
But when the striking surface is narrow, it concentrates the force and makes it easier to chip/dig out those little bits. It's easier to get better sparks with less force/velocity than when you have a wide striking surface. So even when I do use a wide faced striker, I tend to tilt it to one side so that I am striking the flint along just one edge.
Of course, this is just what I have observed over the years of making and using traditional flint strikers. Then we move on to starting with steel that has a high enough carbon content to begin with, and then heat-treating hard enough to work well as a striker. The harder it is heat-treated, the easier it is to chip/dig out those little bits of steel to create your sparks. But that also makes it almost as brittle as you can get it, and more susceptable to breaking. So you have to work out something of a balancing act in how you heat treat your striker.
The manufacturers of files have already worked all of this out when they make their files. It needs to be heat-treated as hard as they can get it - so that it will "cut" other metal. But it also needs to be tough enough not to break/shatter ... too easily. So as long as you don't heat that section of an old file up too hot grinding it, you retain the original heat-treat. And that works well as a flint striker.
Just some humble thoughts to share. Take them as such.
Mikey - that grumpy German blacksmith out in the Hinterlands
