On Windows, a file on a smb share can be references using a file url like:

file://hostname/share-name/file-path/file.ext

QDesktopServices::openUrl() fails to open such a file due to a simple bug in QDesktopServices::openDocument().
For the above example QUrl gives a local filename of
"//hostname/share-name/file-path/file.ext"
which is basically correct but it seems that for remote files ShellExecute cannot cope with '//'.
If the path is converted using toNativeSeparators() before passing it to ShellExecute it just works.