Is there an easy way to dynamically change the extension attached to the file name when a user changes the "save as" file type?

For example:
        With SaveFileDialog1
            .FileName = label1.Text & ".txt"
            .DefaultExt = ".txt"
            .FilterIndex = 1
            .Filter = "TXT File (*.txt)|*.txt| CSV File (*.csv)|*.csv|"
        End With
When the user changes the save type from the default ".txt" to ".csv" the filename remains [I]filename[/].txt rather than the expected filename.csv

Thanks in advance.