You can filtre out the filenames that are .doc or .rtf with LinQ like this:-
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
Hope this helps