For what it's worth, when they still made good stuff Nicholson Ferrier's rasps were metallurgically different from metal-work files.
A file used to shape metal is, well, file-hard all the way through. Which means it is not too hard to break it in half.
As they explained to me, now & again the Ferrier has to get the horse's attention. This he does by rapping him on the foot with his rasp. Were the rasp the same metal as a file, it would break sometimes, disrupting that particular communication.
So a rasp, at least a Ferrier's rasp, was made of a medium carbon steel. At that time it was AISI 1035. This stuff has a nominal 0.35% carbon, as opposed to the nominal 0.95% carbon in the AISI 1095 used for US-made Nicholson files since about 1980.
This medium carbon steel is tougher, that is it will take more of a pounding without breaking than would a 1095 rasp.
But the teeth would not keep an edge quite so well. So, until Our Gov't put an end to it, they coated the rasps well with "cyanide loaf" to give it a very light case.
Don't know what is done today.
I suppose the only practical use for this knowledge is that if you intend to make knives out of old files - use rasps for throwing knives, things that must take a beating, for rough treatment. A knife to hold an edge, or a wood working tool, would be better made from an old Nicholson metal-cutting file.
