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.
Related to XLS Icon
Related icons are the icons with matching tags, as well as all files icons. All icons are in the same flat style. You can either download free PNG icons or buy SVG vectors