diff -u b/core/modules/image/src/Entity/ImageStyle.php b/core/modules/image/src/Entity/ImageStyle.php
--- b/core/modules/image/src/Entity/ImageStyle.php
+++ b/core/modules/image/src/Entity/ImageStyle.php
@@ -180,12 +180,15 @@
       $path = $uri;
       $scheme = file_default_scheme();
     }
+
+    // Check the extension of the derivative.
     $extension = pathinfo($path, PATHINFO_EXTENSION);
     $original_extension = $extension;
     $this->getDerivativeExtension($extension);
     if ($original_extension != $extension) {
       $path .= '.' . $extension;
     }
+
     return $scheme . '://styles/' . $this->id() . '/' . $scheme . '/' . $path;
   }
 
@@ -335,6 +338,14 @@
    * {@inheritdoc}
    */
   public function getPathToken($uri) {
+    // Check the extension of the derivative.
+    $extension = pathinfo($uri, PATHINFO_EXTENSION);
+    $original_extension = $extension;
+    $this->getDerivativeExtension($extension);
+    if ($original_extension != $extension) {
+      $uri .= '.' . $extension;
+    }
+
     // Return the first 8 characters.
     return substr(Crypt::hmacBase64($this->id() . ':' . $uri, \Drupal::service('private_key')->get() . Settings::getHashSalt()), 0, 8);
   }
diff -u b/core/modules/image/src/Plugin/ImageEffect/ConvertImageEffect.php b/core/modules/image/src/Plugin/ImageEffect/ConvertImageEffect.php
--- b/core/modules/image/src/Plugin/ImageEffect/ConvertImageEffect.php
+++ b/core/modules/image/src/Plugin/ImageEffect/ConvertImageEffect.php
@@ -26,6 +26,12 @@
   /**
    * {@inheritdoc}
    */
+  public function transformDimensions(array &$dimensions) {
+  }
+
+  /**
+   * {@inheritdoc}
+   */
   public function applyEffect(ImageInterface $image) {
     if (!$image->convert($this->configuration['format'])) {
       $this->logger->error('Image convert failed using the %toolkit toolkit on %path (%mimetype)', array('%toolkit' => $image->getToolkitId(), '%path' => $image->getSource(), '%mimetype' => $image->getMimeType()));
@@ -38,7 +44,8 @@
    * {@inheritdoc}
    */
   public function getDerivativeMimeType(&$mime_type) {
-    $mime_type = \Drupal::service('file.mime_type.guesser.extension')->guess($this->configuration['format']);
+    // Guess needs a filename.
+    $mime_type = \Drupal::service('file.mime_type.guesser.extension')->guess('test.' . $this->configuration['format']);
   }
 
   /**