Triangular files
— triangular files are mainly used for inside corners. The triangle in the cross section of such a tool is equilateral, so that the angle between the faces is 60°; the processed angle can be more, but not less (otherwise the file simply will not fit in it).
Theoretically, it is possible to use such a file as a replacement for a flat file, but in fact it makes sense to do this only in the most extreme cases — triangular tools in such an application are less convenient and do not allow you to apply significant effort.