diff --git a/core/lib/Drupal/Core/File/MimeType/ExtensionMimeTypeGuesser.php b/core/lib/Drupal/Core/File/MimeType/ExtensionMimeTypeGuesser.php
index dc92133..ae816da 100644
--- a/core/lib/Drupal/Core/File/MimeType/ExtensionMimeTypeGuesser.php
+++ b/core/lib/Drupal/Core/File/MimeType/ExtensionMimeTypeGuesser.php
@@ -18,6 +18,13 @@ class ExtensionMimeTypeGuesser implements MimeTypeGuesserInterface {
   protected $mapper;
 
   /**
+   * The module handler.
+   *
+   * @var \Drupal\Core\Extension\ModuleHandlerInterface
+   */
+  protected $moduleHandler;
+
+  /**
    * Constructs a new ExtensionMimeTypeGuesser.
    *
    * @param \Drupal\Core\Extension\ModuleHandlerInterface $module_handler
@@ -57,7 +64,8 @@ public function guess($path) {
    * Sets the mimetypes/extension mapping to use when guessing mimetype.
    *
    * @param array|null $mapping
-   *   Passing a NULL mapping will cause guess() to use self::$defaultMapping.
+   *   Passing a NULL mapping will cause guess() to use the mapping provided
+   *   by the file.mime_type.mapper service.
    */
   public function setMapping(array $mapping = NULL) {
     $this->getMapper()->setMapping($mapping);
@@ -66,6 +74,8 @@ public function setMapping(array $mapping = NULL) {
   /**
    * Returns the MIME type mapper service.
    *
+   * @todo in D9, properly inject this service.
+   *
    * @return \Drupal\Core\File\MimeType\MimeTypeMapperInterface
    *   The MIME type mapper service.
    */
@@ -76,4 +86,23 @@ protected function getMapper() {
     return $this->mapper;
   }
 
+  /**
+   * Returns ::$mapping or ::$defaultMapping from the mapper.
+   *
+   * ::$mapping and ::$defaultMapping were removed in 8.3.x., here we are using
+   * a magic method to return their value from the mapper service.
+   *
+   * @todo remove in D9, introduced for BC in D8.3.x.
+   */
+  public function __get($name) {
+    switch ($name) {
+      case 'mapping':
+        return $this->getMapper()->_getMapping();
+
+      case 'defaultMapping':
+        return $this->getMapper()->_getDefaultMapping();
+
+    }
+  }
+
 }
diff --git a/core/lib/Drupal/Core/File/MimeType/MimeTypeMapper.php b/core/lib/Drupal/Core/File/MimeType/MimeTypeMapper.php
index db7966f..b528cb0 100644
--- a/core/lib/Drupal/Core/File/MimeType/MimeTypeMapper.php
+++ b/core/lib/Drupal/Core/File/MimeType/MimeTypeMapper.php
@@ -10,6 +10,16 @@
 class MimeTypeMapper implements MimeTypeMapperInterface {
 
   /**
+   * Default MIME extension mapping.
+   *
+   * @todo remove in D9, introduced for BC in D8.3.x.
+   *
+   * @var array
+   *   Array of mimetypes correlated to the extensions that relate to them.
+   */
+  protected $defaultMapping;
+
+  /**
    * MIME extension mapping.
    *
    * @var array
@@ -866,6 +876,51 @@ class MimeTypeMapper implements MimeTypeMapperInterface {
   protected $isMappingAltered = FALSE;
 
   /**
+   * Constructs a new MimeTypeMapper object.
+   *
+   * @todo remove in D9, introduced for BC in D8.3.x.
+   */
+  public function __construct() {
+    $this->defaultMapping = $this->mapping;
+  }
+
+  /**
+   * Returns the ALTERED mapping from extensions to MIME types.
+   *
+   * @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.
+   *
+   * @todo remove in D9, introduced for BC in D8.3.x.
+   *
+   * @internal
+   */
+  public function _getMapping() {
+    return $this->mapping;
+  }
+
+  /**
+   * Returns the DEFAULT mapping from extensions to MIME types, before alter.
+   *
+   * @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.
+   *
+   * @todo remove in D9, introduced for BC in D8.3.x.
+   *
+   * @internal
+   */
+  public function _getDefaultMapping() {
+    return $this->defaultMapping;
+  }
+
+  /**
    * {@inheritdoc}
    */
   public function alterMapping(ModuleHandlerInterface $module_handler) {