Private Function Ext_toLower(ByVal Datei As String)
        Dim tmp, ext As String
        If File.Exists(Datei) Then
            tmp = Path.GetExtension(Datei)
            If Not tmp = ".s" Or Not tmp = ".sd" Or Not tmp = ".wag" Or Not tmp _
              = ".eng" Then
                tmp = tmp.ToLower
                ext = Path.ChangeExtension(Datei, tmp)
                File.Move(Datei, ext)
                Datei = ext
            End If
            Return Datei
        End If
    End Function

If tmp <> ".s" Or tmp <> ".sd" Or tmp <> ".wag" Or tmp <> ".eng" Then
'Anweisung wie oben
End If