Removes the specified formatted data associated to the dragged item at the specified position from the current drag-and-drop operation.
Note: The dataTransfer object and its mozClearDataAt method are supported in Firefox from version 3.5.
The mozClearDataAt method is an extension of the clearData method for dragging multiple items.
In Firefox, the modification of the dragged data is not allowed during the drag operation only at the start point of a drag operation.
That's why, the mozClearDataAt and mozSetDataAt methods can only be called in a handler for the ondragstart event.
The call to the mozClearDataAt and mozSetDataAt methods in other cases raises an exception.