参考

http://www.cnblogs.com/babycool/p/3531696.html

mark 真实文件类型

Dim dlg As new OpenFileDialog
If dlg.ShowDialog = DialogResult.OK Then
    Dim stream = new io.FileStream(dlg.FileName, io.FileMode.Open, io.FileAccess.Read)
    Dim reader As New io.BinaryReader(stream)
    Dim fileclass As String = ""
   
    Try
        For i As Integer = 0 To 1
            fileclass += reader.ReadByte().ToString()
        Next
    Catch generatedExceptionName As Exception
        msgbox("出错")
    End Try
   
    Select Case fileclass
        Case "255216"
            msgbox("jpg")
        Case "239187"
            msgbox("txt")
        Case "208207"
            msgbox("office")
    End Select
   
end if

1、msgbox(fileclass) 弹出什么？

2、本来是什么后缀名的文件？错误判断成了什么？

3、文件发上来测试。