Cheers guys! I was considering making my own tri-corner fret files from regular triangle files, but decided to buy some cheap Amazon "luthier" files instead. I ended up buying "Homyl Fret Crowning File Custom Ground Triangular File for Guitar Luthier Tool, 4/5inch" which is a set of 2 custom ground files.
They turned up today, and despite being obviously slightly bent in transit one of them is massively rusted. And neither have a safe edge ground, so I'm not sure what "custom ground" means. I ended up getting a refund but yeah, pay crap money get crap tools I guess.
Seeing as I got a refund and they don't want their literal garbage back, I'm going to try and straighten them and grind safe edges in. Wire wheel on the bench grinder should take care of the rust.