public string DefaultExt {get; set;}

SaveItemDialog saveMapFileDialog = new SaveItemDialog()
{
  Title = "Save Map File",
  InitialLocation = @"C:\Data\NewYork\Counties\Maps",
  DefaultExt = @"mapx",
  Filter = ItemFilters.Maps_All,
  OverwritePrompt = true
};
bool? result = saveMapFileDialog.ShowDialog();

if (result == true)
{
  ArcGIS.Desktop.Framework.Dialogs.MessageBox.Show("Returned file name: " + saveMapFileDialog.FilePath);
}