diff -u b/core/modules/image/lib/Drupal/image/ImageEffectBase.php b/core/modules/image/lib/Drupal/image/ImageEffectBase.php
--- b/core/modules/image/lib/Drupal/image/ImageEffectBase.php
+++ b/core/modules/image/lib/Drupal/image/ImageEffectBase.php
@@ -48,7 +48,6 @@
    * {@inheritdoc}
    */
   public function transformMimeType(&$mime_type) {
-    $mime_type = NULL;
   }
  
   /**
reverted:
--- b/core/modules/image/lib/Drupal/image/Plugin/ImageEffect/DesaturateImageEffect.php
+++ a/core/modules/image/lib/Drupal/image/Plugin/ImageEffect/DesaturateImageEffect.php
@@ -30,12 +30,6 @@
   /**
    * {@inheritdoc}
    */
-  public function transformMimeType(&$mime_type) {
-  }
-
-  /**
-   * {@inheritdoc}
-   */
   public function applyEffect(ImageInterface $image) {
     if (!$image->desaturate()) {
       watchdog('image', 'Image desaturate failed using the %toolkit toolkit on %path (%mimetype, %dimensions)', array('%toolkit' => $image->getToolkitId(), '%path' => $image->getSource(), '%mimetype' => $image->getMimeType(), '%dimensions' => $image->getWidth() . 'x' . $image->getHeight()), WATCHDOG_ERROR);
reverted:
--- b/core/modules/image/lib/Drupal/image/Plugin/ImageEffect/ResizeImageEffect.php
+++ a/core/modules/image/lib/Drupal/image/Plugin/ImageEffect/ResizeImageEffect.php
@@ -45,12 +45,6 @@
   /**
    * {@inheritdoc}
    */
-  public function transformMimeType(&$mime_type) {
-  }
-
-  /**
-   * {@inheritdoc}
-   */
   public function getSummary() {
     return array(
       '#theme' => 'image_resize_summary',
reverted:
--- b/core/modules/image/lib/Drupal/image/Plugin/ImageEffect/RotateImageEffect.php
+++ a/core/modules/image/lib/Drupal/image/Plugin/ImageEffect/RotateImageEffect.php
@@ -73,12 +73,6 @@
   /**
    * {@inheritdoc}
    */
-  public function transformMimeType(&$mime_type) {
-  }
-
-  /**
-   * {@inheritdoc}
-   */
   public function getSummary() {
     return array(
       '#theme' => 'image_rotate_summary',
reverted:
--- b/core/modules/image/lib/Drupal/image/Plugin/ImageEffect/ScaleImageEffect.php
+++ a/core/modules/image/lib/Drupal/image/Plugin/ImageEffect/ScaleImageEffect.php
@@ -44,12 +44,6 @@
   /**
    * {@inheritdoc}
    */
-  public function transformMimeType(&$mime_type) {
-  }
-
-  /**
-   * {@inheritdoc}
-   */
   public function getSummary() {
     return array(
       '#theme' => 'image_scale_summary',