上传文件判断类型的问题

public enum FileExtension
        {
            JPG = 255216,
            GIF = 7173,
            PNG = 13780,
            SWF = 6787,
            RAR = 8297,
            ZIP = 8075,
            _7Z = 55122,
            BMP=6677,
            EXE=7790,
            XML=6063,
            HTML=6033,
            ASPX=239187,
            CS=117115,
            JS=119105,
            TXT=102100

        }
        public static bool IsAllowedExtension(HttpPostedFile fu, FileExtension[] fileEx)
        {
            int fileLen = fu.ContentLength;
            byte[] imgArray = new byte[fileLen];
            fu.InputStream.Read(imgArray, 0, fileLen);
            MemoryStream ms = new MemoryStream(imgArray);
            System.IO.BinaryReader br = new System.IO.BinaryReader(ms);
            string fileclass = "";
            byte buffer;
            try
            {
                buffer = br.ReadByte();
                fileclass = buffer.ToString();
                buffer = br.ReadByte();
                fileclass += buffer.ToString();
            }
            catch
            {
            }
            br.Close();
            ms.Close();
            foreach (FileExtension fe in fileEx)
            {
                if (Int32.Parse(fileclass) == (int)fe)
                    return true;
            }
            return false;
        }

 if (postedfile != null)
                {
                    if (!Tools.Verification.IsAllowedExtension(postedfile, fe))
                    {
                        if (target != "")
                        {
                            if (System.IO.File.Exists(HttpContext.Current.Server.MapPath(target)))
                            {
                                System.IO.File.Delete(HttpContext.Current.Server.MapPath(target));
                                err = "文件非法！！！";
                            }
                        }
                    }
                }