diff --git a/core/lib/Drupal/Core/File/MimeType/ExtensionMimeTypeGuesser.php b/core/lib/Drupal/Core/File/MimeType/ExtensionMimeTypeGuesser.php
index c6f5eaf..9119048 100644
--- a/core/lib/Drupal/Core/File/MimeType/ExtensionMimeTypeGuesser.php
+++ b/core/lib/Drupal/Core/File/MimeType/ExtensionMimeTypeGuesser.php
@@ -35,7 +35,6 @@ public function __construct(MimeTypeMapperInterface $mapper) {
    * {@inheritdoc}
    */
   public function guess($path) {
-    $mapping = $this->mapper->getMapping();
     $extension = '';
     $file_parts = explode('.', drupal_basename($path));
 
@@ -49,8 +48,9 @@ public function guess($path) {
     //   - awesome.image.jpeg
     while ($additional_part = array_pop($file_parts)) {
       $extension = strtolower($additional_part . ($extension ? '.' . $extension : ''));
-      if (isset($mapping['extensions'][$extension])) {
-        return $mapping['mimetypes'][$mapping['extensions'][$extension]];
+      $mime_type = $this->mapper->getMimeTypeForExtension($extension);
+      if ($mime_type) {
+        return $mime_type;
       }
     }
 
diff --git a/core/lib/Drupal/Core/File/MimeType/MimeTypeMapper.php b/core/lib/Drupal/Core/File/MimeType/MimeTypeMapper.php
index 1f0299d..01e75b9 100644
--- a/core/lib/Drupal/Core/File/MimeType/MimeTypeMapper.php
+++ b/core/lib/Drupal/Core/File/MimeType/MimeTypeMapper.php
@@ -865,24 +865,30 @@ class MimeTypeMapper implements MimeTypeMapperInterface {
   /**
    * {@inheritdoc}
    */
-  public function getMapping() {
-    return $this->mapping;
+  public function addMapping($mimetype, $extension) {
+    if (!in_array($mimetype, $this->mapping['mimetypes'])) {
+      $this->mapping['mimetypes'][] = $mimetype;
+    }
+    $key = array_search($mimetype, $this->mapping['mimetypes']);
+    $this->mapping['extensions'][$extension] = $key;
+
+    return TRUE;
   }
 
   /**
    * {@inheritdoc}
    */
   public function getMimeTypes() {
-    list($mimetypes,) = $this->getMapping();
-    return $mimetypes;
+    return array_values($this->mapping['mimetypes']);
   }
 
   /**
    * {@inheritdoc}
    */
   public function getMimeTypeForExtension($extension) {
-    list($mimetypes, $extensions) = $this->getMapping();
     $extension = strtolower($extension);
-    return isset($extensions[$extension]) ? $mimetypes[$extensions[$extension]] : NULL;
+    $extensions = $this->mapping['extensions'];
+    return isset($extensions[$extension]) ? $this->mapping['mimetypes'][$extensions[$extension]] : NULL;
   }
+
 }
diff --git a/core/lib/Drupal/Core/File/MimeType/MimeTypeMapperInterface.php b/core/lib/Drupal/Core/File/MimeType/MimeTypeMapperInterface.php
index c348193..8618a86 100644
--- a/core/lib/Drupal/Core/File/MimeType/MimeTypeMapperInterface.php
+++ b/core/lib/Drupal/Core/File/MimeType/MimeTypeMapperInterface.php
@@ -10,25 +10,27 @@
  * Provides a sensible mapping between filename extensions and MIME types.
  */
 interface MimeTypeMapperInterface {
+
   /**
-   * Returns known MIME types.
+   * Adds a mapping between a mimetype and a extension.
    *
-   * @return string[]
-   *   An indexed array of MIME types.
+   * @param string $mimetype
+   *   The mimetype the passed extension should map.
+   * @param string $extension
+   *   The extension that should map to the passed mimetype.
+   *
+   * @return bool
+   *   Boolean TRUE if the mapping was added successfully, FALSE otherwise.
    */
-  public function getMimeTypes();
+  public function addMapping($mimetype, $extension);
 
   /**
-   * Returns a mapping from file extensions to appropriate MIME types.
+   * Returns known 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.
+   * @return string[]
+   *   An array of MIME types.
    */
-  public function getMapping();
+  public function getMimeTypes();
 
   /**
    * Returns the appropriate MIME type for a given file extension.
@@ -40,4 +42,5 @@ public function getMapping();
    *   A matching MIME type, or NULL if no MIME type matches the extension.
    */
   public function getMimeTypeForExtension($extension);
+
 }