diff --git a/core/modules/media/src/Plugin/media/Source/OEmbed.php b/core/modules/media/src/Plugin/media/Source/OEmbed.php
index e7417c0018..d7a61984b0 100644
--- a/core/modules/media/src/Plugin/media/Source/OEmbed.php
+++ b/core/modules/media/src/Plugin/media/Source/OEmbed.php
@@ -12,6 +12,7 @@
 use Drupal\Core\File\Exception\FileException;
 use Drupal\Core\File\FileSystemInterface;
 use Drupal\Core\Form\FormStateInterface;
+use Drupal\Core\Image\ImageFactory;
 use Drupal\Core\Messenger\MessengerInterface;
 use Drupal\Core\Url;
 use Drupal\media\IFrameUrlHelper;
@@ -26,6 +27,7 @@
 use GuzzleHttp\Exception\TransferException;
 use Psr\Log\LoggerInterface;
 use Symfony\Component\DependencyInjection\ContainerInterface;
+use Symfony\Component\Mime\MimeTypes;
 
 /**
  * Provides a media source plugin for oEmbed resources.
@@ -123,6 +125,13 @@ class OEmbed extends MediaSourceBase implements OEmbedInterface {
    */
   protected $fileSystem;
 
+  /**
+   * The image factory.
+   *
+   * @var \Drupal\Core\Image\ImageFactory
+   */
+  protected $imageFactory;
+
   /**
    * Constructs a new OEmbed instance.
    *
@@ -154,8 +163,10 @@ class OEmbed extends MediaSourceBase implements OEmbedInterface {
    *   The iFrame URL helper service.
    * @param \Drupal\Core\File\FileSystemInterface $file_system
    *   The file system.
+   * @param \Drupal\Core\Image\ImageFactory $image_factory
+   *   The image factory.
    */
-  public function __construct(array $configuration, $plugin_id, $plugin_definition, EntityTypeManagerInterface $entity_type_manager, EntityFieldManagerInterface $entity_field_manager, ConfigFactoryInterface $config_factory, FieldTypePluginManagerInterface $field_type_manager, LoggerInterface $logger, MessengerInterface $messenger, ClientInterface $http_client, ResourceFetcherInterface $resource_fetcher, UrlResolverInterface $url_resolver, IFrameUrlHelper $iframe_url_helper, FileSystemInterface $file_system) {
+  public function __construct(array $configuration, $plugin_id, $plugin_definition, EntityTypeManagerInterface $entity_type_manager, EntityFieldManagerInterface $entity_field_manager, ConfigFactoryInterface $config_factory, FieldTypePluginManagerInterface $field_type_manager, LoggerInterface $logger, MessengerInterface $messenger, ClientInterface $http_client, ResourceFetcherInterface $resource_fetcher, UrlResolverInterface $url_resolver, IFrameUrlHelper $iframe_url_helper, FileSystemInterface $file_system, ImageFactory $image_factory = NULL) {
     parent::__construct($configuration, $plugin_id, $plugin_definition, $entity_type_manager, $entity_field_manager, $field_type_manager, $config_factory);
     $this->logger = $logger;
     $this->messenger = $messenger;
@@ -164,6 +175,7 @@ public function __construct(array $configuration, $plugin_id, $plugin_definition
     $this->urlResolver = $url_resolver;
     $this->iFrameUrlHelper = $iframe_url_helper;
     $this->fileSystem = $file_system;
+    $this->imageFactory = $image_factory ?: \Drupal::service('image.factory');
   }
 
   /**
@@ -184,7 +196,8 @@ public static function create(ContainerInterface $container, array $configuratio
       $container->get('media.oembed.resource_fetcher'),
       $container->get('media.oembed.url_resolver'),
       $container->get('media.oembed.iframe_url_helper'),
-      $container->get('file_system')
+      $container->get('file_system'),
+      $container->get('image.factory')
     );
   }
 
@@ -390,14 +403,10 @@ protected function getLocalThumbnailUri(Resource $resource) {
     }
     $remote_thumbnail_url = $remote_thumbnail_url->toString();
 
-    // Remove the query string, since we do not want to include it in the local
-    // thumbnail URI.
-    $local_thumbnail_url = parse_url($remote_thumbnail_url, PHP_URL_PATH);
-
     // Compute the local thumbnail URI, regardless of whether or not it exists.
     $configuration = $this->getConfiguration();
     $directory = $configuration['thumbnails_directory'];
-    $local_thumbnail_uri = "$directory/" . Crypt::hashBase64($local_thumbnail_url) . '.' . pathinfo($local_thumbnail_url, PATHINFO_EXTENSION);
+    $local_thumbnail_uri = "$directory/" . Crypt::hashBase64($remote_thumbnail_url) . '.' . $this->getThumbnailFileExtensionFromUrl($remote_thumbnail_url);
 
     // If the local thumbnail already exists, return its URI.
     if (file_exists($local_thumbnail_uri)) {
@@ -415,7 +424,7 @@ protected function getLocalThumbnailUri(Resource $resource) {
     }
 
     try {
-      $response = $this->httpClient->get($remote_thumbnail_url);
+      $response = $this->httpClient->request('GET', $remote_thumbnail_url);
       if ($response->getStatusCode() === 200) {
         $this->fileSystem->saveData((string) $response->getBody(), $local_thumbnail_uri, FileSystemInterface::EXISTS_REPLACE);
         return $local_thumbnail_uri;
@@ -432,6 +441,44 @@ protected function getLocalThumbnailUri(Resource $resource) {
     return NULL;
   }
 
+  /**
+   * Tries to determine the file extension of a thumbnail, based on its URL.
+   *
+   * @param string $thumbnail_url
+   *   The remote URL of the thumbnail.
+   *
+   * @return string|null
+   *   The file extension, or NULL if it could not be determined.
+   */
+  protected function getThumbnailFileExtensionFromUrl(string $thumbnail_url): ?string {
+    // First, try to glean the extension by seeing if the URL path ends with one
+    // of the file extensions supported by the current image toolkit.
+    $path = parse_url($thumbnail_url, PHP_URL_PATH);
+    if ($path) {
+      $extension = strtolower(pathinfo($path, PATHINFO_EXTENSION));
+      $supported_extensions = array_map('strtolower', $this->imageFactory->getSupportedExtensions());
+
+      if (in_array($extension, $supported_extensions, TRUE)) {
+        return $extension;
+      }
+    }
+
+    // If the URL didn't give us any clues about the file extension, make a HEAD
+    // request to the thumbnail URL and see if the headers will give us a MIME
+    // type.
+    $response = $this->httpClient->request('HEAD', $thumbnail_url);
+    if ($response->hasHeader('Content-Type')) {
+      $content_type = $response->getHeader('Content-Type');
+      $extensions = MimeTypes::getDefault()->getExtensions(reset($content_type));
+
+      if ($extensions) {
+        return reset($extensions);
+      }
+    }
+
+    return NULL;
+  }
+
   /**
    * {@inheritdoc}
    */
diff --git a/core/modules/media/tests/fixtures/oembed/video_collegehumor.xml b/core/modules/media/tests/fixtures/oembed/video_collegehumor.xml
index 76815ed138..0894cafcf9 100644
--- a/core/modules/media/tests/fixtures/oembed/video_collegehumor.xml
+++ b/core/modules/media/tests/fixtures/oembed/video_collegehumor.xml
@@ -12,7 +12,10 @@
   <height>343</height>
   <html><h1>By the power of Grayskull, CollegeHumor works!</h1>
   </html>
-  <thumbnail_url>internal:/core/misc/druplicon.png</thumbnail_url>
+  <!-- The thumbnail URL does not contain a file extension, so we use
+  this to test the oEmbed source plugin's thumbnail handling;
+  see \Drupal\Tests\media\FunctionalJavascript\MediaSourceOEmbedVideoTest. -->
+  <thumbnail_url>internal:/media_test_oembed/thumbnail</thumbnail_url>
   <thumbnail_width>88</thumbnail_width>
   <thumbnail_height>100</thumbnail_height>
 </oembed>
diff --git a/core/modules/media/tests/modules/media_test_oembed/media_test_oembed.routing.yml b/core/modules/media/tests/modules/media_test_oembed/media_test_oembed.routing.yml
index 75ce685af7..dca946e656 100644
--- a/core/modules/media/tests/modules/media_test_oembed/media_test_oembed.routing.yml
+++ b/core/modules/media/tests/modules/media_test_oembed/media_test_oembed.routing.yml
@@ -4,3 +4,9 @@ media_test_oembed.resource.get:
     _controller: '\Drupal\media_test_oembed\Controller\ResourceController::get'
   requirements:
     _access: 'TRUE'
+media_test_oembed.resource.thumbnail:
+  path: '/media_test_oembed/thumbnail'
+  defaults:
+    _controller: '\Drupal\media_test_oembed\Controller\ResourceController::getThumbnailWithNoExtension'
+  requirements:
+    _access: 'TRUE'
diff --git a/core/modules/media/tests/modules/media_test_oembed/src/Controller/ResourceController.php b/core/modules/media/tests/modules/media_test_oembed/src/Controller/ResourceController.php
index ab58e2f962..6ea9a1e535 100644
--- a/core/modules/media/tests/modules/media_test_oembed/src/Controller/ResourceController.php
+++ b/core/modules/media/tests/modules/media_test_oembed/src/Controller/ResourceController.php
@@ -2,6 +2,7 @@
 
 namespace Drupal\media_test_oembed\Controller;
 
+use Symfony\Component\HttpFoundation\BinaryFileResponse;
 use Symfony\Component\HttpFoundation\Request;
 use Symfony\Component\HttpFoundation\Response;
 
@@ -30,12 +31,24 @@ public function get(Request $request) {
     else {
       $content = file_get_contents($resources[$asset_url]);
       $response = new Response($content);
-      $response->headers->set('Content-Type', 'application/json');
+      $response->headers->set('Content-Type', 'application/' . pathinfo($resources[$asset_url], PATHINFO_EXTENSION));
     }
 
     return $response;
   }
 
+  /**
+   * Returns an example thumbnail file without an extension.
+   *
+   * @return \Symfony\Component\HttpFoundation\BinaryFileResponse
+   *   The response.
+   */
+  public function getThumbnailWithNoExtension() {
+    $response = new BinaryFileResponse('core/misc/druplicon.png');
+    $response->headers->set('Content-Type', 'image/png');
+    return $response;
+  }
+
   /**
    * Maps an asset URL to a local fixture representing its oEmbed resource.
    *
diff --git a/core/modules/media/tests/src/FunctionalJavascript/MediaSourceOEmbedVideoTest.php b/core/modules/media/tests/src/FunctionalJavascript/MediaSourceOEmbedVideoTest.php
index a5c19ad0c7..a1dbe6e8b1 100644
--- a/core/modules/media/tests/src/FunctionalJavascript/MediaSourceOEmbedVideoTest.php
+++ b/core/modules/media/tests/src/FunctionalJavascript/MediaSourceOEmbedVideoTest.php
@@ -4,6 +4,7 @@
 
 use Drupal\Core\Session\AccountInterface;
 use Drupal\media\Entity\Media;
+use Drupal\media\Entity\MediaType;
 use Drupal\media_test_oembed\Controller\ResourceController;
 use Drupal\Tests\media\Traits\OEmbedTestTrait;
 use Drupal\user\Entity\Role;
@@ -164,6 +165,29 @@ public function testMediaOEmbedVideoSource() {
 
     $assert_session->pageTextContains('The CollegeHumor provider is not allowed.');
 
+    // Register a CollegeHumor video as a second oEmbed resource. Note that its
+    // thumbnail URL does not have a file extension.
+    $media_type = MediaType::load($media_type_id);
+    $source_configuration = $media_type->getSource()->getConfiguration();
+    $source_configuration['providers'][] = 'CollegeHumor';
+    $media_type->getSource()->setConfiguration($source_configuration);
+    $media_type->save();
+    $video_url = 'http://www.collegehumor.com/video/40003213/let-not-get-a-drink-sometime';
+    ResourceController::setResourceUrl($video_url, $this->getFixturesDirectory() . '/video_collegehumor.xml');
+
+    // Create a new media item using a CollegeHumor video.
+    $this->drupalGet("media/add/$media_type_id");
+    $assert_session->fieldExists('Remote video URL')->setValue($video_url);
+    $assert_session->buttonExists('Save')->press();
+
+    /** @var \Drupal\media\MediaInterface $media */
+    $media = Media::load(2);
+    $thumbnail = $media->getSource()->getMetadata($media, 'thumbnail_uri');
+    $this->assertFileExists($thumbnail);
+    // Although the resource's thumbnail URL doesn't have a file extension, we
+    // should have deduced the correct one.
+    $this->assertStringEndsWith('.png', $thumbnail);
+
     // Test anonymous access to media via iframe.
     $this->drupalLogout();
 
diff --git a/core/modules/media/tests/src/Kernel/OEmbedSourceTest.php b/core/modules/media/tests/src/Kernel/OEmbedSourceTest.php
index 923a9cc00f..58a65a42bd 100644
--- a/core/modules/media/tests/src/Kernel/OEmbedSourceTest.php
+++ b/core/modules/media/tests/src/Kernel/OEmbedSourceTest.php
@@ -63,9 +63,11 @@ public function testLocalThumbnailUriQueryStringIsIgnored() {
     // The source plugin will try to fetch the remote thumbnail, so mock the
     // HTTP client to ensure that request returns an empty "OK" response.
     $http_client = $this->prophesize(Client::class);
-    $http_client->get(Argument::type('string'))->willReturn(new Response());
+    $http_client->request('GET', Argument::type('string'))->willReturn(new Response());
     $this->container->set('http_client', $http_client->reveal());
 
+    $http_client->get($thumbnail_url->toString())->willReturn(new Response());
+
     // Mock the resource fetcher so that it will return our mocked resource.
     $resource_fetcher = $this->prophesize(ResourceFetcherInterface::class);
     $resource_fetcher->fetchResource(NULL)->willReturn($resource->reveal());
@@ -83,8 +85,103 @@ public function testLocalThumbnailUriQueryStringIsIgnored() {
     // Get the local thumbnail URI and ensure that it does not contain any
     // query string.
     $local_thumbnail_uri = $media_type->getSource()->getMetadata($media, 'thumbnail_uri');
-    $expected_uri = 'public://oembed_thumbnails/' . Crypt::hashBase64('/core/misc/druplicon.png') . '.png';
+    $expected_uri = 'public://oembed_thumbnails/' . Crypt::hashBase64($thumbnail_url->toString()) . '.png';
     $this->assertSame($expected_uri, $local_thumbnail_uri);
   }
 
+  /**
+   * Data provider for ::testGetThumbnailUri().
+   *
+   * @return array
+   *   The sets of arguments to pass to the test method.
+   */
+  public function providerGetThumbnailUri() {
+    return [
+      'with file extension' => [
+        'https://upload.wikimedia.org/wikipedia/commons/9/90/Dries_Buytaert_at_FOSDEM_2008_by_Wikinews.jpg',
+      ],
+      'no file extension' => [
+        'http://placekitten.com/200/300',
+      ],
+    ];
+  }
+
+  /**
+   * Tests remote thumbnail URL handling.
+   *
+   * @param string $thumbnail_url
+   *   The remote URL of the thumbnail.
+   *
+   * @dataProvider providerGetThumbnailUri
+   */
+  public function testGetThumbnailUri($thumbnail_url) {
+    // We will need a media type that uses the oEmbed plugin.
+    $media = Media::create([
+      'bundle' => $this->createMediaType('oembed:video')->id(),
+      'field_media_oembed_video' => 'test',
+    ]);
+
+    // Mock the services which are used to fetch the resource and thumbnail.
+    $url_resolver = $this->prophesize('\Drupal\media\OEmbed\UrlResolverInterface');
+    $resource_fetcher = $this->prophesize('\Drupal\media\OEmbed\ResourceFetcherInterface');
+    $http_client = $this->prophesize('\GuzzleHttp\ClientInterface');
+
+    $resource = $this->prophesize('\Drupal\media\OEmbed\Resource');
+    $resource->getThumbnailUrl()->willReturn(Url::fromUri($thumbnail_url));
+
+    $url_resolver->getResourceUrl('test')->willReturnArgument(0);
+    $resource_fetcher->fetchResource('test')->willReturn($resource->reveal());
+
+    // The HTTP client should make a HEAD request ONLY if the thumbnail URL
+    // doesn't have a file extension.
+    if (pathinfo($thumbnail_url, PATHINFO_EXTENSION)) {
+      $http_client->request('HEAD', $thumbnail_url)->shouldNotBeCalled();
+    }
+    else {
+      $response = new Response(200, [
+        'Content-Type' => [
+          'image/jpeg',
+        ],
+      ]);
+      $http_client->request('HEAD', $thumbnail_url)
+        // A HEAD request should be made every time the thumbnail is requested
+        // when it lacks a file extension.
+        ->shouldBeCalledTimes(2)
+        ->willReturn($response);
+    }
+    // We always expect the actual thumbnail to be retrieved but a GET
+    // request should only be made ONCE when first downloading the file.
+    $http_client->request('GET', $thumbnail_url)
+      ->shouldBeCalledOnce()
+      ->willReturn(new Response());
+
+    $plugin = new OEmbed(
+      ['source_field' => 'field_media_oembed_video'],
+      'oembed',
+      [],
+      $this->container->get('entity_type.manager'),
+      $this->container->get('entity_field.manager'),
+      $this->container->get('config.factory'),
+      $this->container->get('plugin.manager.field.field_type'),
+      $this->container->get('logger.factory')->get('media'),
+      $this->container->get('messenger'),
+      $http_client->reveal(),
+      $resource_fetcher->reveal(),
+      $url_resolver->reveal(),
+      $this->container->get('media.oembed.iframe_url_helper'),
+      $this->container->get('file_system')
+    );
+
+    // Get the local thumbnail URI twice, to ensure that it is only downloaded
+    // (i.e., with a GET request) once. If the thumbnail URL is remote, we
+    // expect a HEAD request to be made each time by
+    // getRemoteThumbnailPathExtension().
+    for ($i = 0; $i < 2; $i++) {
+      $local_thumbnail_uri = $plugin->getMetadata($media, 'thumbnail_uri');
+      $this->assertNotEmpty($local_thumbnail_uri);
+      $this->assertFileExists($local_thumbnail_uri);
+      $this->assertMatchesRegularExpression('/.jpe?g$/i', $local_thumbnail_uri);
+    }
+  }
+
 }