Göldo Fret Crowning File - professional file for crowning (radius) the frets on tempered string instruments. It offers 2 abrasive surfaces 3 mm wide, ideal for most types of frets, except for thin vintage frets and those of mandolin/ukulele. Length including the wooden handle is 25 cm, width 16 mm.
I bought it thinking I would get something cheap for now to see if it works, and later buy something 'professional', but in the end, there's no need for anything else, it does its job very well.
I used it for stainless steel frets.