docs: def _create_related_attachment(self, filename, content, mimetype=None):
          """
          Convert the filename, content, mimetype triple into a MIME attachment
          object. Adjust headers to use Content-ID where applicable.
          Taken from http://code.djangoproject.com/ticket/4771
          """
          attachment = super(EmailMultiRelated, self)._create_attachment(filename, content, mimetype)
          if filename:
              mimetype = attachment['Content-Type']
              del(attachment['Content-Type'])
              del(attachment['Content-Disposition'])
              attachment.add_header('Content-Disposition', 'inline', filename=filename)
              attachment.add_header('Content-Type', mimetype, name=filename)
              attachment.add_header('Content-ID', '<%s>' % filename)
          return attachment