Drag and drop improvement in Windows environment

UNC path files can be attached to draft mode by drag and drop.

In Windows environment, when you drag and drop UNC path files (e.g. files in network computers) to mew draft mode, it can not attach.
Only non-UNC path files (e.g. files in local file systems) can be attached by drag and drop.

However, in other modes, the UNC path files can be opened by drag and drop.
So, I read emacs lisp/dnd.el and hacked this patch.