I have a slot (cut in the case of a fully assembled computer) about 90mm x
10mm. I need to extend the slot to about 94mm x 10mm.

Is there a tool that would do this cleanly, without making any metal filings
such as would be caused by a metal sawblade or drill bit?

If necessary, I could get away with making two 4mm-long slots at one end of
the existing hole, and just bending the metal between the two slots, instead
of cutting it away.

The clearance available for getting any tool into position is a 6mm wide
strip around the slot perimeter.

I want to avoid metal filings, so I can avoid having to take electrical
parts out of the computer case.