TransportBuilder unable to send emails of Content-Type "text/plain" as of 2.3.3

[twoonesixdigital]

Preconditions (*)

1. Magento Community Edition 2.3.3
2. One or more Magento email templates configured as type "text" via an email_templates.xml file.

Steps to reproduce (*)

1.Attempt to send a template based email of type "text" using Magento\Framework\Mail\Template\TransportBuilder that is of Content-Type "text/plain" using the TransportBuilder class's sendMessage method.

Expected result (*)

1. An email is dispatched whose "Content-Type" header is set to "text/plain"

Actual result (*)

1. The email is sent with a "Content-Type" header set to "text/html"

Additional Information (*)

It appears that as of Magento 2.3.3, it is no longer possible to send "text/plain" emails using the TransportBuilder. This is because, as of 2.3.3, the TransportBuilder's prepareMessage method does not pass the determined template type to the mimePartInterfaceFactory's create method.

Current method definition as of 2.3.3:

protected function prepareMessage()
    {
        $template = $this->getTemplate();
        $content = $template->processTemplate();
        switch ($template->getType()) {
            case TemplateTypesInterface::TYPE_TEXT:
                $part['type'] = MimeInterface::TYPE_TEXT;
                break;

            case TemplateTypesInterface::TYPE_HTML:
                $part['type'] = MimeInterface::TYPE_HTML;
                break;

            default:
                throw new LocalizedException(
                    new Phrase('Unknown template type')
                );
        }
        $mimePart = $this->mimePartInterfaceFactory->create(['content' => $content]);
        $this->messageData['body'] = $this->mimeMessageInterfaceFactory->create(
            ['parts' => [$mimePart]]
        );

        $this->messageData['subject'] = html_entity_decode(
            (string)$template->getSubject(),
            ENT_QUOTES
        );
        $this->message = $this->emailMessageInterfaceFactory->create($this->messageData);

        return $this;
    }

Proposed change that would resolve issue (only the relevant snippet):

$mimePart = $this->mimePartInterfaceFactory->create(
        	[
        		'content' => $content,
        		'type' => $part['type']
        	]
        );

(Pull request incoming)

[m2-assistant]

Hi @twoonesixdigital. Thank you for your report.
To help us process this issue please make sure that you provided the following information:

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• Information on your environment
• Steps to reproduce
• Expected and actual results

Please make sure that the issue is reproducible on the vanilla Magento instance following Steps to reproduce. To deploy vanilla Magento instance on our environment, please, add a comment to the issue:

@magento give me 2.4-develop instance - upcoming 2.4.x release

For more details, please, review the Magento Contributor Assistant documentation.

@twoonesixdigital do you confirm that you were able to reproduce the issue on vanilla Magento instance following steps to reproduce?

• yes
• no

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[twoonesixdigital]

@magento yes

[twoonesixdigital]

Pull request submitted: #26474

[m2-assistant]

Hi @thiaramus. Thank you for working on this issue.
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[thiaramus]

The internal Magento team is working on this issue right now.
The fix will be based on commits from the author and delivered according to our internal processes. All original commits made by the author will be preserved.