SaveFileDialog on Macs

I'm using the SaveFileDialog to allow the user to save a file from the application.  This works great on Windows machines, but on Macs, the default file extension is not being added, so the user has to type it in manually in order for the file to be functional.  Here's the code I'm using.  Does anyone have any thoughts on why this doesn't work on Macs?

SaveFileDialog dialog = new SaveFileDialog();

dialog.Filter = "All Files|*.*";
dialog.DefaultFileName = string.Format("{0}.{1}", item.Name, item.Extension);
dialog.DefaultExt = item.Extension.

bool? success = dialog.ShowDialog();