//真正判断文件类型的关键函数

    public static bool IsAllowedExtension2(FileUpload hifile)

    {

        if (hifile != null)

        {

            System.IO.FileStream fs = new System.IO.FileStream(hifile.PostedFile.FileName, System.IO.FileMode.Open, System.IO.FileAccess.Read);

            System.IO.BinaryReader r = new System.IO.BinaryReader(fs);

            string fileclass = "";

            byte buffer;

            try

            {

                buffer = r.ReadByte();

                fileclass = buffer.ToString();

                buffer = r.ReadByte();

                fileclass += buffer.ToString();

            }

            catch

            {

            }

            r.Close();

            fs.Close();

            if (fileclass == "255216" || fileclass == "7173")//说明255216是jpg;7173是gif;6677是BMP,13780是PNG;7790是exe,8297是rar

            {

                return true;

            }

            else

            {

                return false;

            }

        }

        else { return false; }

    }