diff --git a/core/lib/Drupal/Core/File/MimeType/MimeTypeMapper.php b/core/lib/Drupal/Core/File/MimeType/MimeTypeMapper.php
index 680e94d..ac68b2d 100644
--- a/core/lib/Drupal/Core/File/MimeType/MimeTypeMapper.php
+++ b/core/lib/Drupal/Core/File/MimeType/MimeTypeMapper.php
@@ -898,6 +898,7 @@ protected function getMapping() {
    * {@inheritdoc}
    */
   public function addMapping($mimetype, $extension) {
+    $extension = strtolower($extension);
     if (!in_array($mimetype, $this->mapping['mimetypes'])) {
       $this->mapping['mimetypes'][] = $mimetype;
     }
diff --git a/core/modules/responsive_image/responsive_image.module b/core/modules/responsive_image/responsive_image.module
index f7e2192..929feaa 100644
--- a/core/modules/responsive_image/responsive_image.module
+++ b/core/modules/responsive_image/responsive_image.module
@@ -459,10 +459,8 @@ function responsive_image_get_mime_type($image_style_name, $extension) {
   if ($image_style_name == RESPONSIVE_IMAGE_EMPTY_IMAGE) {
     return 'image/gif';
   }
-  // The MIME type guesser needs a full path, not just an extension, but the
-  // file doesn't have to exist.
-  $fake_path = 'responsive_image.' . ImageStyle::load($image_style_name)->getDerivativeExtension($extension);
-  return Drupal::service('file.mime_type.guesser.extension')->guess($fake_path);
+  $extension = ImageStyle::load($image_style_name)->getDerivativeExtension($extension);
+  return Drupal::service('file.mime_type.mapper')->getMimeTypeForExtension($extension);
 }
 
 /**