diff --git a/core/modules/rest/tests/modules/rest_test/rest_test.services.yml b/core/modules/rest/tests/modules/rest_test/rest_test.services.yml
new file mode 100644
index 0000000..40b5002
--- /dev/null
+++ b/core/modules/rest/tests/modules/rest_test/rest_test.services.yml
@@ -0,0 +1,5 @@
+services:
+  rest_test.format_provider:
+    class: Drupal\rest_test\EventSubscriber\FormatProvider
+    tags:
+      - { name: event_subscriber }
diff --git a/core/modules/rest/tests/modules/rest_test/src/EventSubscriber/FormatProvider.php b/core/modules/rest/tests/modules/rest_test/src/EventSubscriber/FormatProvider.php
new file mode 100644
index 0000000..ac6f237
--- /dev/null
+++ b/core/modules/rest/tests/modules/rest_test/src/EventSubscriber/FormatProvider.php
@@ -0,0 +1,31 @@
+<?php
+
+namespace Drupal\rest_test\EventSubscriber;
+
+use Symfony\Component\EventDispatcher\EventSubscriberInterface;
+use Symfony\Component\HttpKernel\Event\GetResponseEvent;
+use Symfony\Component\HttpKernel\KernelEvents;
+
+/**
+ * Event subscriber for adding additional content types to the request.
+ */
+class FormatProvider implements EventSubscriberInterface {
+
+  /**
+   * Register content type formats on the request object.
+   *
+   * @param \Symfony\Component\HttpKernel\Event\GetResponseEvent $event
+   *   The Event to process.
+   */
+  public function onKernelRequest(GetResponseEvent $event) {
+    $event->getRequest()->setFormat('foobar', ['text/foobar']);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  static function getSubscribedEvents() {
+    $events[KernelEvents::REQUEST][] = ['onKernelRequest'];
+    return $events;
+  }
+}
diff --git a/core/modules/rest/tests/src/Functional/EntityResource/EntityResourceTestBase.php b/core/modules/rest/tests/src/Functional/EntityResource/EntityResourceTestBase.php
index 483923e..f2ed502 100644
--- a/core/modules/rest/tests/src/Functional/EntityResource/EntityResourceTestBase.php
+++ b/core/modules/rest/tests/src/Functional/EntityResource/EntityResourceTestBase.php
@@ -601,15 +601,15 @@ public function testPost() {
     $this->assertAuthenticationEdgeCases('POST', $url, $request_options);
 
 
-    // $request_options[RequestOptions::HEADERS]['Content-Type'] = 'text/xml';
+    $request_options[RequestOptions::HEADERS]['Content-Type'] = 'text/foobar';
 
 
     // DX: 415 when request body in existing but not allowed format.
-    // $response = $this->request('POST', $url, $request_options);
+    $response = $this->request('POST', $url, $request_options);
     // @todo Update this in https://www.drupal.org/node/2826407. Also move it
     // higher, before the "no request body" test. That's impossible right now,
     // because the format validation happens too late.
-    // $this->assertResourceErrorResponse(415, '', $response);
+    $this->assertResourceErrorResponse(415, '', $response);
 
 
     $request_options[RequestOptions::HEADERS]['Content-Type'] = static::$mimeType;
@@ -809,15 +809,15 @@ public function testPatch() {
     $this->assertAuthenticationEdgeCases('PATCH', $url, $request_options);
 
 
-    // $request_options[RequestOptions::HEADERS]['Content-Type'] = 'text/xml';
+    $request_options[RequestOptions::HEADERS]['Content-Type'] = 'text/foobar';
 
 
     // DX: 415 when request body in existing but not allowed format.
-    // $response = $this->request('PATCH', $url, $request_options);
+    $response = $this->request('PATCH', $url, $request_options);
     // @todo Update this in https://www.drupal.org/node/2826407. Also move it
     // higher, before the "no request body" test. That's impossible right now,
     // because the format validation happens too late.
-    // $this->assertResourceErrorResponse(415, '', $response);
+    $this->assertResourceErrorResponse(415, '', $response);
 
 
     $request_options[RequestOptions::HEADERS]['Content-Type'] = static::$mimeType;