diff --git a/core/lib/Drupal/Core/File/MimeType/MimeTypeMapper.php b/core/lib/Drupal/Core/File/MimeType/MimeTypeMapper.php
index bdb34e5..e6553b9 100644
--- a/core/lib/Drupal/Core/File/MimeType/MimeTypeMapper.php
+++ b/core/lib/Drupal/Core/File/MimeType/MimeTypeMapper.php
@@ -863,13 +863,6 @@ class MimeTypeMapper implements MimeTypeMapperInterface {
   );
 
   /**
-   * Track if hook_file_mimetype_mapping_alter() has been already invoked.
-   *
-   * @var bool
-   */
-  protected $isMappingAltered = FALSE;
-
-  /**
    * The module handler.
    *
    * @var \Drupal\Core\Extension\ModuleHandlerInterface
@@ -879,34 +872,30 @@ class MimeTypeMapper implements MimeTypeMapperInterface {
   /**
    * Constructs a new MimeTypeMapper.
    *
+   * Invokes hook_file_mimetype_mapping_alter() to allow modules to alter
+   * the mapping.
+   *
    * @param \Drupal\Core\Extension\ModuleHandlerInterface $module_handler
    *   The module handler.
+   *
+   * @see hook_file_mimetype_mapping_alter()
    */
   public function __construct(ModuleHandlerInterface $module_handler) {
     $this->moduleHandler = $module_handler;
+    $this->moduleHandler->alter('file_mimetype_mapping', $this);
   }
 
   /**
    * Returns the mapping from file extensions to appropriate MIME types.
    *
-   * Invokes hook_file_mimetype_mapping_alter() to allow modules to alter
-   * the mapping.
-   *
    * @return array
    *   An array consisting of two arrays:
    *     - mimetypes: MIME types, keyed by a unique number.
    *     - extensions: an associative array with the MIME type key numbers as
    *       values. The keys are file extensions, in lower case and without any
    *       preceding dot.
-   *
-   * @see hook_file_mimetype_mapping_alter()
    */
   protected function getMapping() {
-    if (!$this->isMappingAltered) {
-      // Allow modules to alter the default mapping.
-      $this->isMappingAltered = TRUE;
-      $this->moduleHandler->alter('file_mimetype_mapping', $this);
-    }
     return $this->mapping;
   }
 
@@ -914,17 +903,13 @@ protected function getMapping() {
    * {@inheritdoc}
    */
   public function addMapping($mimetype, $extension) {
-    // Calls ::getMapping() to make sure new mappings are added to the
-    // ::$mapping array that has been altered by modules already.
-    $this->getMapping();
-
     if (!in_array($mimetype, $this->mapping['mimetypes'])) {
       $this->mapping['mimetypes'][] = $mimetype;
     }
     $key = array_search($mimetype, $this->mapping['mimetypes']);
     $this->mapping['extensions'][$extension] = $key;
 
-    return TRUE;
+    return $this;
   }
 
   /**
diff --git a/core/lib/Drupal/Core/File/MimeType/MimeTypeMapperInterface.php b/core/lib/Drupal/Core/File/MimeType/MimeTypeMapperInterface.php
index 1143aa5..9b05e8c 100644
--- a/core/lib/Drupal/Core/File/MimeType/MimeTypeMapperInterface.php
+++ b/core/lib/Drupal/Core/File/MimeType/MimeTypeMapperInterface.php
@@ -19,8 +19,7 @@
    * @param string $extension
    *   The extension that should map to the passed mimetype.
    *
-   * @return bool
-   *   Boolean TRUE if the mapping was added successfully, FALSE otherwise.
+   * @return $this
    */
   public function addMapping($mimetype, $extension);
 
diff --git a/core/modules/system/file.api.php b/core/modules/system/file.api.php
index ea13843..ee8b9d2 100644
--- a/core/modules/system/file.api.php
+++ b/core/modules/system/file.api.php
@@ -100,7 +100,7 @@ function hook_file_url_alter(&$uri) {
 }
 
 /**
- * Alter MIME type mappings used to determine MIME type from a file extension.
+ * Alter the mapping of MIME types to file extensions.
  *
  * Invoked by \Drupal\Core\File\MimeType\MimeTypeMapper::getMapping(). It
  * is used to allow modules to add to or modify the default mapping from
diff --git a/core/modules/system/src/Tests/File/MimeTypeTest.php b/core/modules/system/src/Tests/File/MimeTypeTest.php
index 1fec540..357fd77 100644
--- a/core/modules/system/src/Tests/File/MimeTypeTest.php
+++ b/core/modules/system/src/Tests/File/MimeTypeTest.php
@@ -2,7 +2,7 @@
 
 /**
  * @file
- * Contains Drupal\system\Tests\File\MimeTypeTest.
+ * Contains \Drupal\system\Tests\File\MimeTypeTest.
  */
 
 namespace Drupal\system\Tests\File;