title: SendGrid - Image not showing up in HTML email
body: <p>I am using the SendGrid v3 API and C# library (v7) to send an email.
  In my email I have a header which is a png. The header is embedded like this:</p>
  
  <pre><code>&lt;img src="cid:emailheader"/&gt;
  </code></pre>
  
  <p>In the C# code I send the image as an attachment with with the same ContentId</p>
  
  <p>var mail = new Mail(from, subject, to, content);</p>
  
  <pre><code>var headerPath = HttpContext.Current.Server.MapPath("~/Resources/email-header.png");
  
  var attachment = new SendGrid.Helpers.Mail.Attachment();
  attachment.ContentId = "emailheader";
  attachment.Content = Convert.ToBase64String(File.ReadAllBytes(headerPath));
  attachment.Type = "image/png";
  attachment.Filename = "email-header.png";
  mail.AddAttachment(attachment);
  
  var send = sg.client.mail.send.post(requestBody: mail.Get());
  </code></pre>
  
  <p>Yet when I open the email it says the source is not found, even though the image is correctly displayed in the attachment</p>
  
  <p><a href="https://i.stack.imgur.com/C2unN.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/C2unN.png" alt="enter image description here"></a></p>