BeforeDrop

fires before the user has finished dragging a file in the queue and released the mouse button

boolean BeforeDrop(object data,Event e);

data	object	data object
e	Event	a native event object

boolean

false - to block the default action, otherwise true

vault.events.on("BeforeDrop", function(data, events){
    // your logic here
    return false;
});

Details

The data object can contain the following parameters:

start	(string) the id of a file, from which the dragging process has started
source	(string[]) an array with ids of dragged files
target	(string) the id of a potential target file