diff --git a/core/lib/Drupal/Core/File/MimeType/ExtensionMimeTypeGuesser.php b/core/lib/Drupal/Core/File/MimeType/ExtensionMimeTypeGuesser.php
index 06d9568..c6f5eaf 100644
--- a/core/lib/Drupal/Core/File/MimeType/ExtensionMimeTypeGuesser.php
+++ b/core/lib/Drupal/Core/File/MimeType/ExtensionMimeTypeGuesser.php
@@ -17,17 +17,17 @@ class ExtensionMimeTypeGuesser implements MimeTypeGuesserInterface {
   /**
    * The MIME type mapper to use for guessing.
    *
-   * @var \Drupal\Core\File\MimeType\MimeTypeMapper
+   * @var MimeTypeMapperInterface
    */
   protected $mapper;
 
   /**
    * Constructs a new ExtensionMimeTypeGuesser.
    *
-   * @param \Drupal\Core\File\MimeType\MimeTypeMapper $mapper
+   * @param MimeTypeMapperInterface $mapper
    *   A MIME type mapper.
    */
-  public function __construct(MimeTypeMapper $mapper) {
+  public function __construct(MimeTypeMapperInterface $mapper) {
     $this->mapper = $mapper;
   }
 
@@ -35,7 +35,7 @@ public function __construct(MimeTypeMapper $mapper) {
    * {@inheritdoc}
    */
   public function guess($path) {
-    $mapping = $this->getMapper()->getMapping();
+    $mapping = $this->mapper->getMapping();
     $extension = '';
     $file_parts = explode('.', drupal_basename($path));
 
@@ -56,15 +56,4 @@ public function guess($path) {
 
     return 'application/octet-stream';
   }
-
-  /**
-   * Returns the MIME type mapper that is used for guessing.
-   *
-   * @return MimeTypeMapper
-   *   A MIME type mapper.
-   */
-  public function getMapper() {
-    return $this->mapper;
-  }
-
 }
diff --git a/core/lib/Drupal/Core/File/MimeType/MimeTypeMapper.php b/core/lib/Drupal/Core/File/MimeType/MimeTypeMapper.php
index a4ad570..f218061 100644
--- a/core/lib/Drupal/Core/File/MimeType/MimeTypeMapper.php
+++ b/core/lib/Drupal/Core/File/MimeType/MimeTypeMapper.php
@@ -11,7 +11,7 @@
 /**
  * Provides a sensible mapping between filename extensions and MIME types.
  */
-class MimeTypeMapper {
+class MimeTypeMapper implements MimeTypeMapperInterface {
 
   /**
    * Default MIME extension mapping.
@@ -887,17 +887,7 @@ public function __construct(ModuleHandlerInterface $module_handler) {
   }
 
   /**
-   * Returns a mapping from file extensions to appropriate MIME types.
-   *
-   * This will use a default mapping and invoke
-   * hook_file_mimetype_mapping_alter() before returning the result.
-   *
-   * @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.
+   * {@inheritdoc}
    */
   public function getMapping() {
     if ($this->mapping === NULL) {
@@ -910,10 +900,7 @@ public function getMapping() {
   }
 
   /**
-   * Replaces the mimetypes/extension mapping.
-   *
-   * @param array|null $mapping
-   *   Passing a NULL mapping will reset the mapping to self::$defaultMapping.
+   * {@inheritdoc}
    */
   public function setMapping(array $mapping = NULL) {
     $this->mapping = $mapping;
diff --git a/core/lib/Drupal/Core/File/MimeType/MimeTypeMapperInterface.php b/core/lib/Drupal/Core/File/MimeType/MimeTypeMapperInterface.php
new file mode 100644
index 0000000..90718cb
--- /dev/null
+++ b/core/lib/Drupal/Core/File/MimeType/MimeTypeMapperInterface.php
@@ -0,0 +1,36 @@
+<?php
+/**
+ * @file
+ * Contains \Drupal\Core\File\MimeType\MimeTypeMapperInterface.
+ */
+
+namespace Drupal\Core\File\MimeType;
+
+/**
+ * Provides a sensible mapping between filename extensions and MIME types.
+ */
+interface MimeTypeMapperInterface {
+  /**
+   * Replaces the mimetypes/extension mapping.
+   *
+   * @param array|null $mapping
+   *   Passing a NULL mapping will reset to the default mapping.
+   */
+  public function setMapping(array $mapping = NULL);
+
+  /**
+   * Returns a mapping from file extensions to appropriate MIME types.
+   *
+   * Unless the mapping has been replaced by setMapping(), this will use a
+   * default mapping and invoke hook_file_mimetype_mapping_alter() before
+   * returning the result.
+   *
+   * @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.
+   */
+  public function getMapping();
+}
diff --git a/core/modules/system/src/Tests/File/MimeTypeTest.php b/core/modules/system/src/Tests/File/MimeTypeTest.php
index 920030f..33db96a 100644
--- a/core/modules/system/src/Tests/File/MimeTypeTest.php
+++ b/core/modules/system/src/Tests/File/MimeTypeTest.php
@@ -84,7 +84,7 @@ public function testFileMimeTypeDetection() {
       'test.ogg' => 'application/octet-stream',
     );
     $extension_guesser = $this->container->get('file.mime_type.guesser.extension');
-    $extension_guesser->getMapper()->setMapping($mapping);
+    $this->container->get('file.mime_type.mapper')->setMapping($mapping);
 
     foreach ($test_case as $input => $expected) {
       $output = $extension_guesser->guess($input);