// The original HtmlMessage may contain base64 images in the <img> tags.
// <img src="data:image/png;base64,R0lGODlhQABAAMQAAJSWl..." />

const sendEmailWithGmail = ({ to, subject, htmlMessage }) => {
  let htmlBody = htmlMessage;
  const inlineImages = {};

  // Find all base64 image tags in the html message.
  const base64ImageTags = htmlBody.match(/<img src="data:image\/(png|jpeg|gif);base64,([^"]+)"[^>]*>/gm) || [];

  base64ImageTags.forEach((base64ImageTag) => {
    // Extract the base64-encoded image data from the tag.
    const [, format, base64Data] = base64ImageTag.match(/data:image\/(png|jpeg|gif);base64,([^"]+)/);

    // Convert the base64 data to binary.
    const imageByte = Utilities.base64Decode(base64Data);

    // Create a blob containing the image data.
    const imageName = Utilities.getUuid();
    const imageBlob = Utilities.newBlob(imageByte, `image/${format}`, imageName);

    // Replace the base64 image tag with cid: image tag.
    const newImageTag = base64ImageTag.replace(/src="[^"]+"/, `src="cid:${imageName}"`);
    htmlBody = htmlBody.replace(base64ImageTag, newImageTag);

    inlineImages[imageName] = imageBlob;
  });

  MailApp.sendEmail({
    to: to,
    subject: subject,
    htmlBody: htmlBody,
    inlineImages: inlineImages,
  });
};