Are banjo bolts grade 8?

Gravity is your friend. Turn the caliper up so the file is under it. You should not need to remove much.

Actually I would not file on a seal surface unless there is visible damage. A file will leave linear marks that are more prone to leak than concentric marks from rotating tools that made the part.

The line can be easily removed & cleaned - or you can just wrap a rag around it & step on the brakes to flush it... A bit of scotch brite, gripped so that each surface of the banjo is under the abrasive & twist the line so that the scratches are concentric to the bolt bore is the way to go for cleaning the line sealing surface.

I would start with a soft set of washers & all the force you can muster by hand on a single box end wrench to tighten the bolt. Wet the threads & the washers with brake fluid when assembling things.

B.