No need to fear when it comes to lowering the depth of your string slots at the nut. This method is so easy even a child can do it!
BTW, these are not files. These are jeweler's piercing saw blades. You can find them on Amazon by searching: GERMAN STYLE JEWELERS SAW FRAME with 144 ASSORTED JEWELERS SAW BLADES
Too, this video may help:
https://www.youtube.com/watch?v=-4z6zucAtB0