ArcGIS Pro 2.6 API Reference Guide
DefaultExt Property
Example 

ArcGIS.Desktop.Catalog Namespace > SaveItemDialog Class : DefaultExt Property
Gets and sets the default file extension
Syntax
public string DefaultExt {get; set;}
Public Property DefaultExt As String
Remarks
If a file extension is not provided in the Browse dialog box's Name text box, the default file extension specified by this property will be appended to the provided file name. For example, when information is saved to a text file, the default file extension might be "txt".
Example
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);
}
Requirements

Target Platforms: Windows 10, Windows 8.1, Windows 7

See Also

Reference

SaveItemDialog Class
SaveItemDialog Members