Member sfx2::FileDialogHelper::GetMPath () const: : Don't use this method to retrieve the selected files There are file picker which can provide multiple selected file which belong to different folders. As this method always provides the root folder for all selected files this cannot work.
Member sfx2::FileDialogHelper::SetDisplayDirectory (const OUString &rPath): don't use this method. It contains a lot of magic in determining whether the last segment of the given path/URL denotes a file name or a folder, and by definition, it cannot succeed with this magic all the time - there will always be scenarios where it fails.