MC-33700: TransportBuilder unable to send emails of Content-Type "text/plain"

Author: Roman Lytvynenko

dev/tests/integration/testsuite/Magento/Framework/Mail/TransportBuilderTest.php

@@ -9,6 +9,8 @@
 
 use Magento\Email\Model\BackendTemplate;
 use Magento\Email\Model\Template;
+use Magento\Framework\App\TemplateTypesInterface;
+use Magento\Framework\Exception\LocalizedException;
 use Magento\Framework\Mail\Template\TransportBuilder;
 use Magento\Framework\ObjectManagerInterface;
 use Magento\TestFramework\Helper\Bootstrap;
@@ -47,11 +49,26 @@ protected function setUp()
      *
      * @param string|array $email
      * @dataProvider emailDataProvider
-     * @throws \Magento\Framework\Exception\LocalizedException
+     * @throws LocalizedException
      */
     public function testAddToEmail($email)
     {
-        $templateId = $this->template->load('email_exception_fixture', 'template_code')->getId();
+        $template = $this->template->load('email_exception_fixture', 'template_code');
+        $templateId = $template->getId();
+
+        switch ($template->getType()) {
+            case TemplateTypesInterface::TYPE_TEXT:
+                $templateType = MimeInterface::TYPE_TEXT;
+                break;
+
+            case TemplateTypesInterface::TYPE_HTML:
+                $templateType = MimeInterface::TYPE_HTML;
+                break;
+
+            default:
+                $templateType = '';
+                $this->fail('Unsupported Mime Type');
+        }
 
         $this->builder->setTemplateModel(BackendTemplate::class);
 
@@ -62,9 +79,11 @@ public function testAddToEmail($email)
         $this->builder->addTo($email);
 
         /** @var EmailMessage $emailMessage */
-        $emailMessage = $this->builder->getTransport();
+        $emailMessage = $this->builder->getTransport()->getMessage();
+
+        $this->assertContains($templateType, $emailMessage->getHeaders()['Content-Type']);
 
-        $addresses = $emailMessage->getMessage()->getTo();
+        $addresses = $emailMessage->getTo();
 
         $emails = [];
         /** @var Address $toAddress */