code: public function submitAttachment(string $idArticle, Model $attachment, string $field): array
      {
          return json_decode(
              $this->post(
                  $this->getAttachmentEndPoint($idArticle, $field),
                  $this->normalizePayload(
                      $attachment->getCollection()
                  )
              ),
              true
          );
      }
docs: submit each attachment of an article here