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

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