8657 Support UNC paths for file objects on Windows

Before this change, UNC paths as the source for files under Windows were
not supported and caused errors.  This limited the usefulness of the
file type under Windows.  This patch adjusts the logic used to handle
UNC paths so they can be successfully used.
Merge pull request #1888 from pocams/fix/master/f8657

(Feature #8657) Support UNC paths on Windows