If you are buying at Lowes, the selection will be limited to what the "normal" people buy
I like half round files for working radius, but they are not at all common in a hardware store.
Get a couple of the ones you do buy.
then take one to the grinder and grind the small edge smooth.
This is a safe edge and will let you file up into a corner on one side without removing material from both sides.
This is especially helpful on guard fitting in small files
Grind one file safe on the short edge, one edge safe on the flat side.