diff --git a/core/lib/Drupal/Core/File/MimeType/ExtensionMimeTypeGuesser.php b/core/lib/Drupal/Core/File/MimeType/ExtensionMimeTypeGuesser.php
index b0c716a..53712ca 100644
--- a/core/lib/Drupal/Core/File/MimeType/ExtensionMimeTypeGuesser.php
+++ b/core/lib/Drupal/Core/File/MimeType/ExtensionMimeTypeGuesser.php
@@ -11,7 +11,7 @@
 use Symfony\Component\HttpFoundation\File\MimeType\MimeTypeGuesserInterface;
 
 /**
- * Makes possible to guess the MIME type of a stream wrapper path.
+ * Makes possible to guess the MIME type of a file using its extension.
  */
 class ExtensionMimeTypeGuesser implements MimeTypeGuesserInterface {
 
@@ -883,19 +883,16 @@ class ExtensionMimeTypeGuesser implements MimeTypeGuesserInterface {
    *
    * @param \Drupal\Core\Extension\ModuleHandlerInterface $module_handler
    *   The module handler.
-   * @param array $mapping.
-   *   (optional) The mapping to use. If empty self::defaultMapping is used.
    */
-  public function __construct(ModuleHandlerInterface $module_handler, array $mapping = array()) {
+  public function __construct(ModuleHandlerInterface $module_handler) {
     $this->moduleHandler = $module_handler;
-    $this->mapping = $mapping;
   }
 
   /**
    * {@inheritdoc}
    */
   public function guess($path) {
-    if (empty($this->mapping)) {
+    if ($this->mapping === NULL) {
       $mapping = $this->defaultMapping;
       // Allow modules to alter the default mapping.
       $this->moduleHandler->alter('file_mimetype_mapping', $mapping);
@@ -923,4 +920,14 @@ public function guess($path) {
     return 'application/octet-stream';
   }
 
+  /**
+   * 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.
+   */
+  public function setMapping(array $mapping = NULL) {
+    $this->mapping = $mapping;
+  }
+
 }
diff --git a/core/modules/system/lib/Drupal/system/Tests/File/MimeTypeTest.php b/core/modules/system/lib/Drupal/system/Tests/File/MimeTypeTest.php
index 8efc05f..06ef0fb 100644
--- a/core/modules/system/lib/Drupal/system/Tests/File/MimeTypeTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/File/MimeTypeTest.php
@@ -64,7 +64,7 @@ public function testFileMimeTypeDetection() {
       $this->assertIdentical($output, $expected, format_string('Mimetype (using default mappings) for %input is %output (expected: %expected).', array('%input' => $input, '%output' => $output, '%expected' => $expected)));
     }
 
-    // Now test passing in the map.
+    // Now test the extension gusser by passing in a custom mapping.
     $mapping = array(
       'mimetypes' => array(
         0 => 'application/java-archive',
@@ -91,10 +91,11 @@ public function testFileMimeTypeDetection() {
       'foo.doc' => 'application/octet-stream',
       'test.ogg' => 'application/octet-stream',
     );
-    $guesser = new ExtensionMimeTypeGuesser($this->container->get('module_handler'), $mapping);
+    $extension_guesser = $this->container->get('file.mime_type.guesser.extension');
+    $extension_guesser->setMapping($mapping);
 
     foreach ($test_case as $input => $expected) {
-      $output = $guesser->guess($input, $mapping);
+      $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)));
     }
   }