check upload file is zip or not

protected void btn_Click(object sender, EventArgs e)

    {        

        //testFile is an object of File Upload Contol

        if((testFile.FileName.Substring(testFile.FileName.LastIndexOf(".") + 1)).ToLower() = "zip")

            Response.Write("This is a zip file");

        else

            Response.Write("This is not a zip file");

    }