diff --git a/core/lib/Drupal/Core/File/MimeType/MimeTypeMapper.php b/core/lib/Drupal/Core/File/MimeType/MimeTypeMapper.php
index b5b64de..f7c37a3 100644
--- a/core/lib/Drupal/Core/File/MimeType/MimeTypeMapper.php
+++ b/core/lib/Drupal/Core/File/MimeType/MimeTypeMapper.php
@@ -19,7 +19,7 @@ class MimeTypeMapper implements MimeTypeMapperInterface {
    * @var array
    *   Array of mimetypes correlated to the extensions that relate to them.
    */
-  protected $defaultMapping = array(
+  protected $mapping = array(
     'mimetypes' => array(
       0 => 'application/andrew-inset',
       1 => 'application/atom',
@@ -863,11 +863,11 @@ class MimeTypeMapper implements MimeTypeMapperInterface {
   );
 
   /**
-   * The MIME types mapping array after going through the module handler.
+   * Track if hook_file_mimetype_mapping_alter() has been already invoked.
    *
-   * @var array
+   * @var bool
    */
-  protected $mapping;
+  protected $isMappingAltered = FALSE;
 
   /**
    * The module handler.
@@ -890,11 +890,10 @@ public function __construct(ModuleHandlerInterface $module_handler) {
    * {@inheritdoc}
    */
   protected function getMapping() {
-    if ($this->mapping === NULL) {
-      $mapping = $this->defaultMapping;
+    if (!$this->isMappingAltered) {
       // Allow modules to alter the default mapping.
-      $this->moduleHandler->alter('file_mimetype_mapping', $mapping);
-      $this->mapping = $mapping;
+      $this->isMappingAltered = TRUE;
+      $this->moduleHandler->alter('file_mimetype_mapping', $this);
     }
     return $this->mapping;
   }
@@ -903,10 +902,11 @@ protected function getMapping() {
    * {@inheritdoc}
    */
   public function addMapping($mimetype, $extension) {
-    if (!in_array($mimetype, $this->getMapping()['mimetypes'])) {
+    $this->getMapping();
+    if (!in_array($mimetype, $this->mapping['mimetypes'])) {
       $this->mapping['mimetypes'][] = $mimetype;
     }
-    $key = array_search($mimetype, $this->getMapping()['mimetypes']);
+    $key = array_search($mimetype, $this->mapping['mimetypes']);
     $this->mapping['extensions'][$extension] = $key;
 
     return TRUE;
@@ -923,20 +923,22 @@ public function getMimeTypes() {
    * {@inheritdoc}
    */
   public function getMimeTypeForExtension($extension) {
+    $mapping = $this->getMapping();
     $extension = strtolower($extension);
-    $extensions = $this->getMapping()['extensions'];
-    return isset($extensions[$extension]) ? $this->getMapping()['mimetypes'][$extensions[$extension]] : NULL;
+    $extensions = $mapping['extensions'];
+    return isset($extensions[$extension]) ? $mapping['mimetypes'][$extensions[$extension]] : NULL;
   }
 
   /**
    * {@inheritdoc}
    */
   public function getExtensionsForMimeType($mimetype) {
-    if (!in_array($mimetype, $this->getMapping()['mimetypes'])) {
+    $mapping = $this->getMapping();
+    if (!in_array($mimetype, $mapping['mimetypes'])) {
       return [];
     }
-    $key = array_search($mimetype, $this->getMapping()['mimetypes']);
-    $extensions = array_keys($this->getMapping()['extensions'], $key, TRUE);
+    $key = array_search($mimetype, $mapping['mimetypes']);
+    $extensions = array_keys($mapping['extensions'], $key, TRUE);
     sort($extensions);
     return $extensions;
   }
diff --git a/core/modules/file/tests/file_test/file_test.module b/core/modules/file/tests/file_test/file_test.module
index 6ee2ecc..07f76cc 100644
--- a/core/modules/file/tests/file_test/file_test.module
+++ b/core/modules/file/tests/file_test/file_test.module
@@ -9,6 +9,7 @@
  */
 
 use Drupal\file\Entity\File;
+use Drupal\Core\File\MimeType\MimeTypeMapperInterface;
 
 const FILE_URL_TEST_CDN_1 = 'http://cdn1.example.com';
 const FILE_URL_TEST_CDN_2 = 'http://cdn2.example.com';
@@ -286,16 +287,14 @@ function file_test_file_url_alter(&$uri) {
 /**
  * Implements hook_file_mimetype_mapping_alter().
  */
-function file_test_file_mimetype_mapping_alter(&$mapping) {
+function file_test_file_mimetype_mapping_alter(MimeTypeMapperInterface $mime_type_mapper) {
   // Add new mappings.
-  $mapping['mimetypes']['file_test_mimetype_1'] = 'madeup/file_test_1';
-  $mapping['mimetypes']['file_test_mimetype_2'] = 'madeup/file_test_2';
-  $mapping['mimetypes']['file_test_mimetype_3'] = 'madeup/doc';
-  $mapping['extensions']['file_test_1'] = 'file_test_mimetype_1';
-  $mapping['extensions']['file_test_2'] = 'file_test_mimetype_2';
-  $mapping['extensions']['file_test_3'] = 'file_test_mimetype_2';
+  $mime_type_mapper->addMapping('madeup/file_test_1', 'file_test_1');
+  $mime_type_mapper->addMapping('madeup/file_test_2', 'file_test_2');
+  $mime_type_mapper->addMapping('madeup/file_test_2', 'file_test_3');
+  $mime_type_mapper->addMapping('drupal/info', 'info.yml');
   // Override existing mapping.
-  $mapping['extensions']['doc'] = 'file_test_mimetype_3';
+  $mime_type_mapper->addMapping('madeup/doc', 'doc');
 }
 
 /**
diff --git a/core/modules/system/file.api.php b/core/modules/system/file.api.php
index a4fd8e4..135a1de 100644
--- a/core/modules/system/file.api.php
+++ b/core/modules/system/file.api.php
@@ -102,25 +102,30 @@ function hook_file_url_alter(&$uri) {
 /**
  * Alter MIME type mappings used to determine MIME type from a file extension.
  *
- * Invoked by \Drupal\Core\File\MimeType\ExtensionMimeTypeGuesser::guess(). It
+ * Invoked by \Drupal\Core\File\MimeType\MimeTypeMapper::getMapping(). It
  * is used to allow modules to add to or modify the default mapping from
- * \Drupal\Core\File\MimeType\ExtensionMimeTypeGuesser::$defaultMapping.
- *
- * @param $mapping
- *   An array of mimetypes correlated to the extensions that relate to them.
- *   The array has 'mimetypes' and 'extensions' elements, each of which is an
- *   array.
- *
- * @see \Drupal\Core\File\MimeType\ExtensionMimeTypeGuesser::guess()
- * @see \Drupal\Core\File\MimeType\ExtensionMimeTypeGuesser::$defaultMapping
+ * \Drupal\Core\File\MimeType\MimeTypeMapper::$mapping. Implementations
+ * should use the \Drupal\Core\File\MimeType\MimeTypeMapper::addMapping()
+ * method to add/override MIME type to extension mapping. The
+ * \Drupal\Core\File\MimeType\MimeTypeMapper::getMimeTypeForExtension() and
+ * \Drupal\Core\File\MimeType\MimeTypeMapper::getExtensionsForMimeType()
+ * methods can be used to get the current mapping respectively of the MIME
+ * type of a given extension, and of the extensions associated to a given
+ * MIME type.
+ *
+ * @param \Drupal\Core\File\MimeType\MimeTypeMapperInterface $mime_type_mapper
+ *   The MIME type mapper object.
+ *
+ * @see \Drupal\Core\File\MimeType\MimeTypeMapper::addMapping()
+ * @see \Drupal\Core\File\MimeType\MimeTypeMapper::getMimeTypeForExtension()
+ * @see \Drupal\Core\File\MimeType\MimeTypeMapper::getExtensionsForMimeType()
  */
-function hook_file_mimetype_mapping_alter(&$mapping) {
-  // Add new MIME type 'drupal/info'.
-  $mapping['mimetypes']['example_info'] = 'drupal/info';
-  // Add new extension '.info.yml' and map it to the 'drupal/info' MIME type.
-  $mapping['extensions']['info'] = 'example_info';
+function hook_file_mimetype_mapping_alter(\Drupal\Core\File\MimeType\MimeTypeMapperInterface $mime_type_mapper) {
+  // Add new MIME type 'drupal/info', and map it to a new extension
+  // '.info.yml'.
+  $mime_type_mapper->addMapping('drupal/info', 'info.yml');
   // Override existing extension mapping for '.ogg' files.
-  $mapping['extensions']['ogg'] = 189;
+  $mime_type_mapper->addMapping('audio/ogg', 'ogg');
 }
 
 /**
diff --git a/core/modules/system/src/Tests/File/MimeTypeTest.php b/core/modules/system/src/Tests/File/MimeTypeTest.php
index aa7e554..453e02f 100644
--- a/core/modules/system/src/Tests/File/MimeTypeTest.php
+++ b/core/modules/system/src/Tests/File/MimeTypeTest.php
@@ -44,6 +44,7 @@ public function testFileMimeTypeDetection() {
       'foo.file_test_2' => 'madeup/file_test_2',
       'foo.doc' => 'madeup/doc',
       'test.ogg' => 'audio/ogg',
+      'test.info.yml' => 'drupal/info',
     );
 
     $guesser = $this->container->get('file.mime_type.guesser');
@@ -83,6 +84,7 @@ public function testFileMimeTypeDetection() {
       'foo.file_test_2' => 'madeup/file_test_2',
       'foo.doc' => 'madeup/doc',
       'test.ogg' => 'audio/ogg',
+      'test.info.yml' => 'drupal/info',
     );
 
     foreach ($test_case as $input => $expected) {
@@ -99,6 +101,7 @@ public function testFileMimeTypeDetection() {
       'madeup/file_test_1' => ['file_test_1'],
       'madeup/file_test_2' => ['file_test_2', 'file_test_3'],
       'madeup/doc' => ['doc'],
+      'drupal/info' => ['info.yml'],
     ];
     foreach ($test_case as $input => $expected) {
       $extensions = $mime_type_mapper->getExtensionsForMimeType($input);