diff --git a/core/lib/Drupal/Core/File/MimeType/MimeTypeMapper.php b/core/lib/Drupal/Core/File/MimeType/MimeTypeMapper.php
index e6553b9..a375001 100644
--- a/core/lib/Drupal/Core/File/MimeType/MimeTypeMapper.php
+++ b/core/lib/Drupal/Core/File/MimeType/MimeTypeMapper.php
@@ -863,6 +863,15 @@ class MimeTypeMapper implements MimeTypeMapperInterface {
   );
 
   /**
+   * Track if mapping has been altered already.
+   *
+   * @see hook_file_mimetype_mapping_alter()
+   *
+   * @var bool
+   */
+  protected $isMappingAltered = FALSE;
+
+  /**
    * The module handler.
    *
    * @var \Drupal\Core\Extension\ModuleHandlerInterface
@@ -872,30 +881,39 @@ 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() {
+    static $altering = FALSE;
+
+    // Allow modules to alter the default mapping.
+    if (!$this->isMappingAltered && !$altering) {
+      $altering = TRUE;
+      $this->moduleHandler->alter('file_mimetype_mapping', $this);
+      $this->isMappingAltered = TRUE;
+      $altering = FALSE;
+    }
+
     return $this->mapping;
   }
 
@@ -903,6 +921,10 @@ 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;
     }