check this javascript file
/usr/share/cinnamon/applets/window-list@cinnamon.org/applet.js
you should be able to edit that. i'm gonna check myself if i can find it.
if (this.metaWindow.minimized) {
title = "["+ title +"]";
} else if (this.metaWindow.tile_type == Meta.WindowTileType.TILED) {
title = "|"+ title;