Revise rendered content

DataCollector/MessageDataCollector.php

@@ -60,9 +60,17 @@ public function collect(Request $request, Response $response, \Exception $except
                     ];
 
                     foreach ($logger->getMessages() as $message) {
-                        ($userContentTypeProp = new \ReflectionProperty(\Swift_Message::class, 'userContentType'))->setAccessible(true);
-                        $message->__contentType = $userContentTypeProp->getValue($message);
+                        $message->__contentType = $message->getBodyContentType();
                         $message->__base64EncodedBody = base64_encode($message->getBody());
+                        if ('text/plain' === $message->__contentType) {
+                            foreach ($message->getChildren() as $child) {
+                                if ('text/html' === $child->getContentType()) {
+                                    $message->__contentType = 'text/html';
+                                    $message->__base64EncodedBody = base64_encode($child->getBody());
+                                    break;
+                                }
+                            }
+                        }
                         $this->data['mailer'][$name]['messages'][] = $message;
                     }
 

composer.json

@@ -17,7 +17,7 @@
     ],
     "require": {
         "php": ">=7.0.0",
-        "swiftmailer/swiftmailer": "^6.0.1",
+        "swiftmailer/swiftmailer": "^6.1.3",
         "symfony/dependency-injection": "~2.7|~3.3|~4.0",
         "symfony/http-kernel": "~2.7|~3.3|~4.0",
         "symfony/config": "~2.8|~3.3|~4.0"