Add Attachment base64 image in MailMessage

some.cs

        private static AlternateView ContentToAlternateView(string content)
        {
            Stream Base64ToImageStream(string base64String)
            {
                byte[] imageBytes = Convert.FromBase64String(base64String);
                MemoryStream ms = new MemoryStream(imageBytes, 0, imageBytes.Length);
                return ms;
            }

            var imgCount = 0;
            List<LinkedResource> resourceCollection = new List<LinkedResource>();
            foreach (Match m in Regex.Matches(content, @"<img([\s\S]+?)src\s?=\s?['""](?<srcAttr>[^'""]+?)['""]([\s\S]+?)\/?>"))
            {
                imgCount++;
                var srcAttribute = m.Groups["srcAttr"].Value;
                string type = Regex.Match(srcAttribute, ":(?<type>.*?);base64,").Groups["type"].Value;
                string base64 = Regex.Match(srcAttribute, "base64,(?<base64>.*?)$").Groups["base64"].Value;
                
                //ignore replacement when match normal <img> tag
                if (String.IsNullOrEmpty(type) || String.IsNullOrEmpty(base64)) continue;

                content = content.Replace(srcAttribute, "cid:" + imgCount);

                var tempResource = new LinkedResource(Base64ToImageStream(base64), new ContentType(type))
                {
                    ContentId = imgCount.ToString()
                };
                resourceCollection.Add(tempResource);
            }

            AlternateView alternateView = AlternateView.CreateAlternateViewFromString(content, null, MediaTypeNames.Text.Html);
            foreach (var item in resourceCollection)
            {
                alternateView.LinkedResources.Add(item);
            }

            return alternateView;
        }