C# Image attachment isn't shown inline on redemption

I use the following code to add an image to an RDOMessage and try to show this image inside the HTMLBody...

_message.BodyFormat = 2;
_message.HTMLBody = MailHelper.GetHtmlMessage(MailBody, "utf-8");

if (ImageSource != null && !string.IsNullOrWhiteSpace(ImageName))
{
    PngBitmapEncoder encoder = new PngBitmapEncoder();
    encoder.Frames.Add(BitmapFrame.Create(ImageSource));
    string id = "qr-code.png";
    int index = _message.HTMLBody.IndexOf("%imageQr%");
    if (index >= 0)
    {
        string tempDir = TempDirectory.Create();
        string tmpFile = Path.Combine(tempDir, id);
        using (FileStream fileStream = new FileStream(tmpFile, FileMode.Create))
        {
            encoder.Save(fileStream);
        }
                    
        RDOAttachment attachment = _message.Attachments.Add(tmpFile, 1, null, ImageNage);
        attachment.ContentID = id;
        _message.HTMLBody = _message.HTMLBody.Insert(index, $"<img src=\"cid:{id}\">");
        _message.HTMLBody = _message.HTMLBody.Replace("%imageQr%", string.Empty);
    }
}

But the only thing I see in the message is this:

So the question is: What is not correct? (before and after the image are each three underlines, that is not the problem ;)) I used it in a slightly different version with the OOM version and that worked well... Interesting to note is here, that with the OOM I could specify the id as a Guid without file name ending (i.e. without a .png), if I did that here with RDO the attachment itself was also not able to open...

Edit: When I send the email it arrives more or less correctly with this body tag:

<body lang=DE link="#0563C1" vlink="#954F72" style='word-wrap:break-word'>
    <div class=WordSection1>
        <p>
             ___<img width=32 height=32 style='width:.3333in;height:.3333in' id="_x0000_i1025" src="cid:8b2b3585-c611-4562-884c-915fc9f9b759">___
           <o:p>
           </o:p>
        </p>
    </div>
</body>

But the size is not correct and I didn't set it explicitly, also the style is not set by me...

What it should look like is this (and it does with OOM):

______

The image has 100x100 px, so the second would be correct...

But what even more is disturbing is, that it looks like below when I havn't send it yet. The user would think that the image is broken. (And I don't know how to see the HTML of a not sent email).

If I save the message and open it again it looks like this:

Shortly before I send the message the HTMLBody looks like this:

<!DOCTYPE html>
<html>
   <head>
      <meta charset=\"utf-8\">
   </head>
   <body>
      <p>___<img src=\"cid:ab0c3c9a-9a9f-4fe7-a8e6-45c0735473a6\">___</p>
   </body>
</html>

And BodyFormat is 2, so it should be used as HTML, shouldn't it? So, why does it not arrive at Outlook like this?

Solution

With the help of @Dmitry Streblechenko (thanks) I found the answer:

After adding the ID to the attachment I had to save the attachment explicitly. So adding a simple:

attachment.Save();

after

attachment.ContentID = id;

did the trick.