prepareMessageObject

/**
 * Converts inline images and external images into Encoded Data.  Removes converted inline images from "attachments collection"
 * @param {GmailMessage} messageObject A gmail message
 * @return {PreppedMessageObject} result 
 *    @return {GmailMessage} result.messageObject Original gmail message
 *    @return {String} result.cleanBody new message body
 *    @return {Attachment} result.cleanAttachments attachments excluding inline images
 *    @return {String} result.cleanSubject
 *    @return {String} result.cleanDate
 */
function prepMessageObject(messageObject){
  var emailTemplate = messageObject.getBody();
  var rawContent = messageObject.getRawContent();
  var attachments = messageObject.getAttachments();
  var cleanDateString = Utilities.formatDate(messageObject.getDate(), "America/Toronto", "EEE, MMM d, YYYY 'at' h:mm aa");
  var hostedImgToReplace = [];
    var inlineImgToReplace = [];

  var inlineImages = {};

  var imgVars = emailTemplate.match(/<img[^>]+>/g); // retrieve all image tags in body.
  
  var regMessageId = new RegExp(messageObject.getId(), "g"); // RegEx to find the message id in strings
  
    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) inlineImgToReplace.push([imgTitle[1], imgVars[i], id[1]]);
          }
        }
        
        else {
         hostedImgToReplace.push(["No Title",imgVars[i],"No ID"]);         
        }
        
        
      }
    }

    for (var i = 0; i < inlineImgToReplace.length; i++) {
      for (var j = 0; j < attachments.length; j++) {
        if(attachments[j].getName() == inlineImgToReplace[i][0]) {
          
          inlineImages[inlineImgToReplace[i][2]] = attachments[j].copyBlob();
          var newData = "data:" + inlineImages[inlineImgToReplace[i][2]].getContentType()+';base64,'+ Utilities.base64Encode(inlineImages[inlineImgToReplace[i][2]].getBytes()) 
          
          attachments.splice(j, 1);
          if (newData.substr(0,5) == "data:") {
             var newImg = inlineImgToReplace[i][1].replace(/src="[^\"]+\"/, "src=\"" + newData + "\"");
             emailTemplate = emailTemplate.replace(inlineImgToReplace[i][1], newImg);
          }
        }
      }
    }

    for(var i=0;i<hostedImgToReplace.length;i++){
      
      var srcVar = hostedImgToReplace[i][1].match(/(src=\")([^\"]+)(\")/);
      if (srcVar != null){
        var newData = srcToData(srcVar[2]);
        if (newData.substr(0,5) == "data:") {
      
          var newImg = hostedImgToReplace[i][1].replace(/src="[^\"]+\"/, "src=\"" + newData + "\"");
          emailTemplate = emailTemplate.replace(hostedImgToReplace[i][1], newImg);
        }
      }
      
    }  
  

  var result = {
    messageObject: messageObject,
    cleanBody: emailTemplate,
    cleanAttachments: attachments,
    cleanSubject: messageObject.getSubject() == "" ? "(no subject)" : messageObject.getSubject(),
    cleanDateString: cleanDateString,
  }
  return result;
}



function srcToData(string){
  if (string.substr(0,5) == "data:"){return string; }
  var img = UrlFetchApp.fetch(string);
  var data = img.getBlob().getContentType()+';base64,'+ Utilities.base64Encode(img.getBlob().getBytes());
  
  return "data:"+data;
  
  
}