Just a Guy that cuts wood
ArboristSite Guru
plus 1 on rectangular but if someone is looking it up it's called a double beveled chisel file made by PFERDThis is what he is talking about.
http://www.baileysonline.com/Chains...el-Files/Pferd-3-Corner-Chisel-Files-Each.axd
OK if you want to use it in a guide, but if you are free handing, get the rectangular file, just trust me on that!
I exclusively use the rectangular files.