diff --git a/core/lib/Drupal/Core/File/MimeType/MimeTypeMapper.php b/core/lib/Drupal/Core/File/MimeType/MimeTypeMapper.php
index f218061..627b458 100644
--- a/core/lib/Drupal/Core/File/MimeType/MimeTypeMapper.php
+++ b/core/lib/Drupal/Core/File/MimeType/MimeTypeMapper.php
@@ -902,7 +902,19 @@ public function getMapping() {
   /**
    * {@inheritdoc}
    */
-  public function setMapping(array $mapping = NULL) {
-    $this->mapping = $mapping;
+  public function getMimeTypes() {
+    // Fire alter hooks.
+    list($mimetypes,) = $this->getMapping();
+    return $mimetypes;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getMimeTypeForExtension($extension) {
+    // Fire alter hooks.
+    list($mimetypes, $extensions) = $this->getMapping();
+    $extension = strtolower($extension);
+    return isset($extensions[$extension]) ? $mimetypes[$extensions[$extension]] : NULL;
   }
 }
diff --git a/core/lib/Drupal/Core/File/MimeType/MimeTypeMapperInterface.php b/core/lib/Drupal/Core/File/MimeType/MimeTypeMapperInterface.php
index 90718cb..c348193 100644
--- a/core/lib/Drupal/Core/File/MimeType/MimeTypeMapperInterface.php
+++ b/core/lib/Drupal/Core/File/MimeType/MimeTypeMapperInterface.php
@@ -11,20 +11,16 @@
  */
 interface MimeTypeMapperInterface {
   /**
-   * Replaces the mimetypes/extension mapping.
+   * Returns known MIME types.
    *
-   * @param array|null $mapping
-   *   Passing a NULL mapping will reset to the default mapping.
+   * @return string[]
+   *   An indexed array of MIME types.
    */
-  public function setMapping(array $mapping = NULL);
+  public function getMimeTypes();
 
   /**
    * 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-   * Unless the mapping has been replaced by setMapping(), this will use a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.
@@ -33,4 +29,15 @@ public function setMapping(array $mapping = NULL);
    *       preceding dot.
    */
   public function getMapping();
+
+  /**
+   * Returns the appropriate MIME type for a given file extension.
+   *
+   * @param string $extension
+   *   A file extension, without leading dot.
+   *
+   * @return string|null
+   *   A matching MIME type, or NULL if no MIME type matches the extension.
+   */
+  public function getMimeTypeForExtension($extension);
 }
diff --git a/core/modules/system/src/Tests/File/MimeTypeTest.php b/core/modules/system/src/Tests/File/MimeTypeTest.php
index 33db96a..3d21b16 100644
--- a/core/modules/system/src/Tests/File/MimeTypeTest.php
+++ b/core/modules/system/src/Tests/File/MimeTypeTest.php
@@ -2,7 +2,7 @@
 
 /**
  * @file
- * Definition of Drupal\system\Tests\File\MimeTypeTest.
+ * Contains Drupal\system\Tests\File\MimeTypeTest.
  */
 
 namespace Drupal\system\Tests\File;
@@ -55,40 +55,5 @@ public function testFileMimeTypeDetection() {
       $output = $guesser->guess($input);
       $this->assertIdentical($output, $expected, format_string('Mimetype (using default mappings) for %input is %output (expected: %expected).', array('%input' => $input, '%output' => $output, '%expected' => $expected)));
     }
-
-    // Now test the extension gusser by passing in a custom mapping.
-    $mapping = array(
-      'mimetypes' => array(
-        0 => 'application/java-archive',
-        1 => 'image/jpeg',
-      ),
-      'extensions' => array(
-         'jar' => 0,
-         'jpg' => 1,
-      )
-    );
-
-    $test_case = array(
-      'test.jar' => 'application/java-archive',
-      'test.jpeg' => 'application/octet-stream',
-      'test.jpg' => 'image/jpeg',
-      'test.jar.jpg' => 'image/jpeg',
-      'test.jpg.jar' => 'application/java-archive',
-      'test.pcf.z' => 'application/octet-stream',
-      'pcf.z' => 'application/octet-stream',
-      'jar' => 'application/octet-stream',
-      'some.junk' => 'application/octet-stream',
-      'foo.file_test_1' => 'application/octet-stream',
-      'foo.file_test_2' => 'application/octet-stream',
-      'foo.doc' => 'application/octet-stream',
-      'test.ogg' => 'application/octet-stream',
-    );
-    $extension_guesser = $this->container->get('file.mime_type.guesser.extension');
-    $this->container->get('file.mime_type.mapper')->setMapping($mapping);
-
-    foreach ($test_case as $input => $expected) {
-      $output = $extension_guesser->guess($input);
-      $this->assertIdentical($output, $expected, format_string('Mimetype (using passed-in mappings) for %input is %output (expected: %expected).', array('%input' => $input, '%output' => $output, '%expected' => $expected)));
-    }
   }
 }