WM_DROPFILES
hDrop = (HANDLE) wParam; /* handle of internal drop structure */The WM_DROPFILES message is sent when the user releases the left mouse button over the window of an application that has registered itself as a recipient of dropped files.
hDrop
Value of wParam. Identifies an internal data structure describing the dropped files. This handle is used by the DragFinish, DragQueryFile, and DragQueryPoint functions to retrieve information about the dropped files.
An application should return zero if it processes this message.