public function addAttachment($attachment, $group = 'contents')
    {
        if (!$attachment instanceof AttachableInterface && !$attachment instanceof ModelInterface) {
            return false;
        }

        $join = $this->modelFactory()->create(Join::class);

        $objId   = $this->id();
        $objType = $this->objType();
        $attId   = $attachment->id();

        $join->setAttachmentId($attId);
        $join->setObjectId($objId);
        $join->setGroup($group);
        $join->setObjectType($objType);

        $join->save();

        return $this;
    }