Jeweler quality escapement file.  Diamond coated, very fine grit (D64 or U.S. standard 230/270).  5 1/2" in length with a plastic handle that provides a comfortable grip.

We carry the three-square or triangle file which provides a nice scoring edge when cutting filato.  To use the file, simply draw one of the edges across the flat face of the filato.  You are not using the file to cut the filato, only to make a score line.  Then you can easily and accurately snap the filato to the size you want.