Add DFF extension to Direct Stream Digital format

Author: Zeugma440

ATL/AudioData/AudioDataIOFactory.cs

@@ -230,6 +230,7 @@ public static AudioDataIOFactory GetInstance()
                 tempFmt.AddMimeType("audio/x-dsd");
                 tempFmt.AddExtension(".dsf");
                 tempFmt.AddExtension(".dsd");
+                tempFmt.AddExtension(".dff");
                 tempFmt.CheckHeader = DSF.IsValidHeader;
                 theFactory.addFormat(tempFmt);