Public Class Form1
'// [Designerregion]
    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As _
      System.EventArgs) Handles Button1.Click
        For Each f As String In IO.Directory.GetFiles("C:\Winnt")
            Me.ListBox1.Items.Add(New MyFile(f))
        Next
    End Sub
    Private Sub ListBox1_SelectedIndexChanged(ByVal sender As System.Object, _
      ByVal e As System.EventArgs) Handles ListBox1.SelectedIndexChanged
        Dim f As MyFile = CType(Me.ListBox1.SelectedItem, MyFile)
        Dim fullFileName As String = f.File '// Ist der kmpl. Pfad!
        MessageBox.Show("Dateeindung ist: " + New IO.FileInfo( _
          fullFileName).Extension)
    End Sub
End Class
Public Class MyFile : Inherits Object
    Public File As String
    Public Sub New(ByVal File As String)
        Me.File = File
    End Sub
    Public Overrides Function ToString() As String
        Return IO.Path.GetFileNameWithoutExtension(File)
    End Function
End Class

Dim f As MyFile = CType(Me.ListBox1.SelectedItem, MyFile)