The function to call when opening a file on a remote machine.
The function will be called with two arguments; URI and ACTION. See
`dnd-open-file' for details.
If nil, then dragging remote files into Emacs will result in an error.
Predefined functions are `dnd-open-local-file' and `dnd-open-remote-url'.
`dnd-open-local-file' attempts to open a remote file using its UNC name and
is the default on MS-Windows. `dnd-open-remote-url' uses `url-handler-mode'
and is the default except for MS-Windows.