The first step to solving a problem is to diagnose it properly.
Under normal circumstances, properly sized metal posts in metal frames DO NOT SLIP, and even are greased and still do not slip. So there's something wrong.
The OPs reference to widening the slot with a file implies that the top may be touching such that it cannot be tightened farther. That in turn clearly indicates that the seatpost is of the wrong diameter for that frame, and from the extent of excess narrowing of the slot, I wouldn't be at all surprised if the correct seatpost were as large as 27.2mm.
In any case, if the slot is narrowing more than 1mm or so, the OP should confirm the correct post diameter (trial and error with larger posts) before trying to fix what may not be broken.
__________________
FB
Chain-L site
An ounce of diagnosis is worth a pound of cure.
Just because I'm tired of arguing, doesn't mean you're right.
“One accurate measurement is worth a thousand expert opinions” - Adm Grace Murray Hopper - USN
WARNING, I'm from New York. Thin skinned people should maintain safe distance.