Private Sub Command1_Click()
 
Dim oShell As Object
Dim oFolder As Object
Dim file As String, ext As String, i As Integer, fn As String
Set oShell = CreateObject("Shell.Application")
Set oFolder = oShell.BrowseForFolder(Me.hWnd, "Bitte einen Ordner auswählen", 1)
If Not oFolder Is Nothing Then
   file = Dir(oFolder.self.Path & "\*.*")
   While file <> ""
       ext = "": fn = file
       'Extension ermitteln
       For i = Len(file) To 1 Step -1
           If Mid(file, i, 1) = "." Then
              ext = Mid(file, i)
              fn = Mid(file, 1, i - 1)
           End If
       Next i
       'Kleinschreibung Dateiname
       fn = LCase(fn)
       file = fn + ext
       'Umbenennung
       Name oFolder.self.Path & "\" & file As oFolder.self.Path & "\" & file
       file = Dir
   Wend
End If
 
End Sub