Icons are visual representations or metaphors which permits to associate an object, an action or a state to them.
Two principles (how the user see it, click it) :
Views with icons representation should always rely on a square hit test area (which encloses the icon) when the user needs to manipulate them. Don't use something like the alpha channel as the hit area. See Fitt's law.
When you create files icons, you must take in account that the bottom part will probably be covered by a badge, then you must ensure that the key visual part is in the icon superior 2/3 part.