VB如何分辨Winsock发过来的是消息还是文件

Private Sub Listen_DataArrival(Index As IntegerByVal bytesTotal As Long)

Dim Data, b

Dim bytData() As Byte

Dim lLenFile As Long

Dim f, a

Listen.GetData Data

b = Time()

If Data = "212f15r5SDDD511b/',],[D4" Then

   f = FreeFile

   Open FileName For Binary As #f ''FileName是文件名

   lLenFile = LOF(f)

   ReDim bytData(1 To bytesTotal)

   Listen.GetData bytData
   
   Fil (FileName)
   
   If a = 0 Then
   
      MsgBox "文件已存在!"
      
      Exit Sub
      
   End If

   If lLenFile = 0 Then        'lLenFile=0表示是第一次打开文件，这里有个问题，就是如果如果该文件存在的话，就会出错，应该在打开前检查文件是否存在

      Put #f, 1, bytData

   Else

      Put #f, lLenFile + 1, bytData

End If

   Close #f
   
End If

End Sub