Remove an attachment of a Gmail email with Google Apps Script

Using Google Apps Script (http://script.google.com), I know from the docs, how to send, forward, move to trash messages, etc. but I don't find how to remove a file attachement of an email, i.e.:

1. keep the text content (either in HTML or just plain text would be fine)
2. keep the original sender, keep the recipient
3. keep the original message date/hour (important!)
4. remove the attachment

If it's not possible via the API, is there a way to resend the message to myself, while keeping 1, 2 and 3?

Note: the GmailAttachment class looks interesting and allows to list recipients:

var threads = GmailApp.getInboxThreads(0, 10);
 var msgs = GmailApp.getMessagesForThreads(threads);
 for (var i = 0 ; i < msgs.length; i++) {
   for (var j = 0; j < msgs[i].length; j++) {
     var attachments = msgs[i][j].getAttachments();
     for (var k = 0; k < attachments.length; k++) {
       Logger.log('Message "%s" contains the attachment "%s" (%s bytes)',
                  msgs[i][j].getSubject(), attachments[k].getName(), attachments[k].getSize());
     }
   }
 }

but I don't find how to remove an attachment.

Note: I've already studied many other solutions for doing this, I've already read nearly every article about this (solutions with dedicated web services, with local clients like Thunderbird + Attachment extractor plugin, etc.), but none of them are really really cool. That's why I was looking for a solution to do it manually via Google Apps Script.

Looks like messages will have to be re-created-ish:

Messages are immutable: they can only be created and deleted. No message properties can be changed other than the labels applied to a given message.

Using Advanced Gmail Service with the Gmail API insert() you can hack your way around it using: Gmail.Users.Messages.insert(resource, userId)

This advanced service must be enabled before use.

Example: [fill in the EMAIL_ID with an email_id or in whatever way you want to get the email]

function removeAttachments () {
  // Get the `raw` email
  var email = GmailApp.getMessageById("EMAIL_ID").getRawContent();

  // Find the end boundary of html or plain-text email
  var re_html = /(-*\w*)(\r)*(\n)*(?=Content-Type: text\/html;)/.exec(email);
  var re = re_html || /(-*\w*)(\r)*(\n)*(?=Content-Type: text\/plain;)/.exec(email);

  // Find the index of the end of message boundary
  var start = re[1].length + re.index;
  var boundary = email.indexOf(re[1], start);

  // Remove the attachments & Encode the attachment-free RFC 2822 formatted email string
  var base64_encoded_email = Utilities.base64EncodeWebSafe(email.substr(0, boundary));
  // Set the base64Encoded string to the `raw` required property
  var resource = {'raw': base64_encoded_email}

  // Re-insert the email into the user gmail account with the insert time
  /* var response = Gmail.Users.Messages.insert(resource, 'me'); */

  // Re-insert the email with the original date/time 
  var response = Gmail.Users.Messages.insert(resource, 'me', 
                      null, {'internalDateSource': 'dateHeader'});

  Logger.log("The inserted email id is: %s",response.id)
}

This will remove the attachments from the email and re-insert it into your mailbox.

edit/update: New RegExp to work with html&plain-text only emails - should now work on multiple boundary strings