*DON'T* file the shaft - its far to easy to FUBAR it + the shaft must be supported in a V block so you don't stress the bearing or wiper mechanism excessively, and tape used to stop filings and grit getting into the pot. Also, unless you are an expert at precision hand-fitting, you are almost certain to get it out of round, and tapered, convex or concave so the knob wobbles like a friday night drunk.
The knob needs to be bored out to 6.3mm. To do this with hand tools without messing up the knob is really difficult - you really need the help of someone with a lathe or mill, or at least access to a pillar drill + a spare 6.3mm drill bit you can grind the point off to finish the bottom of the hole. 6.4mm is over-size - you'd need to shim the shaft with some very thin metal sheet wrapped round it. Try Aluminum foil tape.