Recently, I needed to shape the inside of some blind (non-through) holes. My first thought was to wrap sandpaper around a dowel, but then I found the perfect tool in round chainsaw files.
They are supplied with a “safe” smooth end that doesn’t cut.
I cut off the ends with a rotary tool, then ground the ends smooth and straight with my bench grinder. Now, the file teeth go right to the end.
For regular work where the file is gripped at both ends, they are no less comfortable to use because the teeth are so fine.