XLS and other 74,400 icons from Icons8 icon pack follow the visual guidelines of the operating systems: Windows, iOS, Android KitKat, and Material. PNG format is free up to 100x100 px. Icons are availble for download in PNG and vector (SVG, PDF, EPS, and more).
It's a rectangular looking shape with the top right corner folded in slightly, so it technically looks like a trapezoid, with a right angle drawn into the top-right corner (if you are picturing a trapezoid). There are three letters on the inside positioned in the center, more so to the bottom than the top, they are X, L, S, in that order.