asp.net 上传判断文件是否为图片

         private bool IsPicture(string filePath)//filePath是文件的完整路径
        {
            try
            {
                FileStream fs = new FileStream(filePath, FileMode.Open, FileAccess.Read);
                BinaryReader reader = new BinaryReader(fs);
                string fileClass;
                byte buffer;
                byte[] b=new byte[2];
                buffer = reader.ReadByte();
                b[0] = buffer;
                fileClass = buffer.ToString();
                buffer = reader.ReadByte();
                b[1]=buffer;
                fileClass += buffer.ToString();
                reader.Close();
                fs.Close();
                if (fileClass == "255216 ")//255216是jpg;7173是gif;6677是