Those files that Cut4fun showed a picture of will keep you filing longer. They don't allow you to give a full stroke, so if you try them plan on spending even more time trying to get a good edge
The 6 sided double bevel old stihl square files were pretty good stuff IMO. You just couldnt get them anymore after say 08 09 time frame anymore.
Skippy, If I might throw in my advice ?? Just get a box of 10 double bevel files as cheap as you can find. You will have to sharpen quite a few loops to get good, and by that time you will go through a few files. Get the feel of a Pferd, Bacho, or Save Edge .............. see if you can get good results. The file doesn't make the cutter cut well, the cutter is made to cut well by the person doing the filing. Maybe Mdavlee or Hedgerow could squeak out a few tenths with a "better" file, but I don't think it would make a difference to most others
Skippy you want a single bevel straight cut valo what ever the ef they are called? I'll give it to u I'll also give you the pos Stihl 3 square that they sell now.