option for selcting .doc or .rtf file in dropdown

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VB

here in the below coding i got the filenames displayed in the listbox, but i need the option for selecting .doc or .rtf which is to in dropdown in text box by selecting the selected fileformats to be displayed in the textbox.........

Using flb As New FolderBrowserDialog
           If flb.ShowDialog() = Windows.Forms.DialogResult.OK Then
               ListBox1.Items.Clear()
               TextBox1.Text = flb.SelectedPath
               Dim fileNames As String() = Directory.GetFiles(flb.SelectedPath)
               Dim selectedFiles As String() = From fileName In fileNames Where fileName.EndsWith(".rtf") Or fileName.EndsWith(".doc") Select fileName
               For Each filename In selectedFiles
                   ListBox1.Items.Add(filename)
               Next
           End If
       End Using