List<LinkedResource> linkedResources;
LinkedResource _attachment = null; 

Guid id = Guid.NewGuid();
        try
        {
            string _tmp = Path.GetExtension(_fileName);

            switch (_tmp.ToUpper())
            {
                case ".XML":
                    _attachment = new LinkedResource(ms, "text/xml");
                    break;
                case ".PNG":
                    _attachment = new LinkedResource(ms, "image/png");
                    break;
                case ".THMX":
                    _attachment = new LinkedResource(ms, "application/vnd.ms-officetheme");
                    break;
                case ".JPG":
                    _attachment = new LinkedResource(ms, "image/jpeg");
                    break;
                case ".GIF":
                    _attachment = new LinkedResource(ms, "image/gif");
                    break;
                case ".SVG":
                    _attachment = new LinkedResource(ms, "image/svg+xml");
                    break;
                case ".TIF":
                    _attachment = new LinkedResource(ms, "image/tiff");
                    break;
                case ".ICO":
                    _attachment = new LinkedResource(ms, "image/vnd.microsoft.icon");
                    break;
                case ".BMP":
                    _attachment = new LinkedResource(ms, "image/bmp");
                    break;
                case ".CSS":
                    _attachment = new LinkedResource(ms, "text/css");
                    break;
                case ".JS":
                    _attachment = new LinkedResource(ms, "application/javascript");
                    break;
                case ".JPEG":
                    _attachment = new LinkedResource(ms, "image/jpeg");
                    break;
                case ".WMZ":
                    _attachment = new LinkedResource(ms, "application/x-ms-wmz");
                    break;
                default: return string.Empty;
            }
            _attachment.ContentId = id.ToString();
            linkedResources.Add(_attachment);
  AlternateView newView = AlternateView.CreateAlternateViewFromString(messageText, null, MediaTypeNames.Text.Html);
                    newView.ContentType = new ContentType("multipart/related");
                    foreach (var item in linkedResources)
                    {
                        newView.LinkedResources.Add(item);
                    }
                    newView.ContentType = new ContentType("text/html");
                    _message.Body = messageText;
                    _message.AlternateViews.Add(newView);
                    _message.IsBodyHtml = true;