As I understand, you can only associate the left, right, middle buttons and the mouse wheel (up, down) with a condition/action. I guess that with online games this should be sufficient and perhaps the 4rth and 5fth mouse buttons might even cause problems with the internet browsers, because they are most commonly used for "go back"/"go forward".
But for PC games with the node-webkit I believe that the two additional buttons offer more versatility and game-play options to the game designers and to the players. I for sure try to use them on every game that I play and get particularly frustrated when the game developers won't give me the option to remap controls to these buttons (I use the XMouseButtonControl).
Is it possible ( Ashley ) to add the additional thumb buttons as an condition in construct 2?