二进制图片格式转换

 public static bool IsAllowedExtension(byte[] imgArray, FileExtension[] fileEx)
        {
            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;
        }
        public  enum FileExtension
        {
            JPG = 255216,
            GIF = 7173,
            PNG = 13780,
            SWF = 6787,
            RAR = 8297,
            ZIP = 8075,
            _7Z = 55122,
            XLS = 208207,
            XLSX = 8075,
            bmp = 6677
        }

调用
 //判断下是否为gif
                        if (IsAllowedExtension(buffer, new FileExtension[] { FileExtension.GIF,FileExtension.PNG,FileExtension.bmp }))
                        {
                            using (MemoryStream newstream = new MemoryStream())
                            {
                                bitmap.Save(newstream, System.Drawing.Imaging.ImageFormat.Jpeg);
                                //
                                using (Bitmap newbitmap = new Bitmap(newstream, true))
                                {
}
}
}