getContentPlain example

if ($value instanceof \stdClass) {
                $value = json_decode((string) json_encode($value), true, 512, \JSON_THROW_ON_ERROR);
            }
        });

        $this->getBrowser()
            ->request(
                'POST',
                '/api/_action/mail-template/send',
                [
                    'contentHtml' => $mailTemplate->getContentHtml(),
                    'contentPlain' => $mailTemplate->getContentPlain(),
                    'mailTemplateData' => [
                        'order' => $orderDecode,
                        'salesChannel' => $salesChannelDecode,
                    ],
                    'documentIds' => $documentIds,
                    'recipients' => ['d.dinh@shopware.com' => 'Duy'],
                    'salesChannelId' => $salesChannel->getId(),
                    'senderName' => $salesChannel->getName(),
                    'subject' => 'New document for your order',
                    'testMode' => false,
                ],
            );