Hello friendly StackOverflow folks,
I am having the most difficult time getting the GmailApp sendEmail() method to successfully use an existing email (e.g. a draft) that contains images inline as a template for new messages.
It seems like this is a problem, as I have found devs having this problem here and here and proposed solutions here, here, and here. Each of these solutions is 4+ years old, so perhaps they're out of date. I have been unable to use any of these solutions to replicate a success.
Currently, I'm running this code from my Google Scripts backend:
function generateMessageFromTemplate () {
  var selectedTemplate = GmailApp.getMessageById('MESSAGE_ID');
  //////////////////////////////////////////////////////////////////////////////
  // Get inline images and make sure they stay as inline images (via Romain Vialard)
  //////////////////////////////////////////////////////////////////////////////
  var emailTemplate = selectedTemplate.getBody();
  var rawContent = selectedTemplate.getRawContent();
  var attachments = selectedTemplate.getAttachments();
  var regMessageId = new RegExp(selectedTemplate.getId(), "g");
  if (emailTemplate.match(regMessageId) != null) {
    var inlineImages = {};
    var nbrOfImg = emailTemplate.match(regMessageId).length;
    var imgVars = emailTemplate.match(/<img[^>]+>/g);
    var imgToReplace = [];
    if(imgVars != null){
      for (var i = 0; i < imgVars.length; i++) {
        if (imgVars[i].search(regMessageId) != -1) {
          var id = imgVars[i].match(/realattid=([^&]+)&/);
          if (id != null) {
            var temp = rawContent.split(id[1])[1];
            temp = temp.substr(temp.lastIndexOf('Content-Type'));
            var imgTitle = temp.match(/name="([^"]+)"/);
            if (imgTitle != null) imgToReplace.push([imgTitle[1], imgVars[i], id[1]]);
          }
        }
      }
    }
    for (var i = 0; i < imgToReplace.length; i++) {
      for (var j = 0; j < attachments.length; j++) {
        if(attachments[j].getName() == imgToReplace[i][0]) {
          inlineImages[imgToReplace[i][2]] = attachments[j].copyBlob();
          attachments.splice(j, 1);
          var newImg = imgToReplace[i][1].replace(/src="[^\"]+\"/, "src=\"cid:" + imgToReplace[i][2] + "\"");
          emailTemplate = emailTemplate.replace(imgToReplace[i][1], newImg);
        }
      }
    }
  }
  //////////////////////////////////////////////////////////////////////////////
    GmailApp.sendEmail('test@email.com', selectedTemplate.getSubject(), '', {
      attachments: attachments,
      htmlBody: emailTemplate,
      inlineImages: inlineImages
    });
};
The Google Scripts documentation on the sendEmail() method is here.
This is the Output of this Function as Is
When I send emails from Apps Script as is, I get emails that look like this: screenshot I've replicated the test with an old yahoo.com email account and had the exact same results as a Gmail account.
Again, this dev also has this same issue.
If you can help, I would be extremely grateful!