I'm trying to find out what kind of format 3dsmax uses for material drag and drop.
I tried to inspect the mime-type and data by dropping Materials from max into a pyQt window, but it didn't receive any drops from max.
Max uses its own internal drag-and-drop system. It can receive drags from explorer because it has a handler that reads the path of the item being dropped, but there is no way it can send data back out in a format that Windows would understand.
What are you trying to achieve specifically? If you want to handle a drop action within max, implement the class I linked, if you want to drag something out of max into an external program... well, that may be possible, but your going to have to do all the heavy lifting yourself.