Issue #3230547 by larowlan, karishmaamin, amjad1233: \Drupal\media\Controller\OEmbedIframeController::render doesn't set a content-type header

core/modules/media/src/Controller/OEmbedIframeController.php

@@ -134,7 +134,9 @@ class OEmbedIframeController implements ContainerInjectionInterface {
     // Return a response instead of a render array so that the frame content
     // will not have all the blocks and page elements normally rendered by
     // Drupal.
-    $response = new HtmlResponse();
+    $response = new HtmlResponse('', HtmlResponse::HTTP_OK, [
+      'Content-Type' => 'text/html; charset=UTF-8',
+    ]);
     $response->addCacheableDependency(Url::createFromRequest($request));
 
     try {

core/modules/media/tests/src/Kernel/OEmbedIframeControllerTest.php

@@ -105,6 +105,7 @@ class OEmbedIframeControllerTest extends MediaKernelTestBase {
     $this->assertStringContainsString('&pasta=rigatoni', $content);
     $this->assertStringContainsString('test.css', $content);
     $this->assertContains('yo_there', $response->getCacheableMetadata()->getCacheTags());
+    $this->assertStringContainsString('text/html', $response->headers->get('Content-Type'));
   }
 
 }