Overview
If you run Rhino elevated (Right click | Run as Administrator), and you try and drag-and-drop a 3DM file from Windows Explorer, nothing will happen. The reason for this is Windows User Account Control (UAC). With UAC enabled, drag and drop from Windows Explorer will not work if Rhino is elevated.
More information
UAC allows you to run processes with different privilege levels on the same desktop. In order to prevent potential elevation of privilege attacks, certain functionality needs to be blocked. This includes Windows messages being sent from process with a lower privilege level to one running at a higher privilege level.
Drag-and-drop is implemented via Windows messages. Therefore, if you try and drag-and-drop a file from Windows Explorer (medium privilege) to Rhino running elevated (high privilege), the Windows messages are blocked and drag-and-drop doesn’t work.
The best solution is to only use drag and drop between the same privilege levels. With UAC enabled, Windows Explorer will run at a medium privilege level. Therefore, Rhion needs to run at medium (or lower) privilege level.