Un'idea potrebbe essere questa :

codice:
Private Sub Command1_Click()
Call fileext(Text1.Text)
End Sub

Function fileext(nomefile As String) As String
Dim ext As String
Dim pos As String

len_string = Len(nomefile)
pos = InStr(1, nomefile, ".")
ext = Mid(nomefile, pos, len_string)
MsgBox ext
End Function