VS.Net2012 does not accept a cshtml file to be dropped on it. And when I right click on the cshtml file and select vs.net 2012 to open, it opens a new instance of the vs.net 2012.
When I test drag-dropping a .cshtml file on a VS 2012 window it opens up in the VS text editior.
So, it should work. Are you running VS 2012 as administrator? In that case drag-drop to open files doesn't work as it is blocked, probably for security reasons. In this case you get a mouse pointer indicating that the drop operation is blocked when trying to drop the file.