diff --git a/core/modules/responsive_image/responsive_image.module b/core/modules/responsive_image/responsive_image.module
index 0d59f57..5519114 100644
--- a/core/modules/responsive_image/responsive_image.module
+++ b/core/modules/responsive_image/responsive_image.module
@@ -472,11 +472,23 @@ function responsive_image_get_image_dimensions($image_style_name, array $dimensi
  *   The MIME type of the image after the image style is applied.
  */
 function responsive_image_get_mime_type($image_style_name, $extension) {
-  if ($image_style_name == RESPONSIVE_IMAGE_EMPTY_IMAGE) {
-    return 'image/gif';
-  }
-  if ($image_style_name !== RESPONSIVE_IMAGE_ORIGINAL_IMAGE) {
-    $extension = ImageStyle::load($image_style_name)->getDerivativeExtension($extension);
+  switch ($image_style_name) {
+    // The file extension for the empty image is 'gif'.
+    case RESPONSIVE_IMAGE_EMPTY_IMAGE:
+      $extension = 'gif';
+      break;
+
+    // If using the original image, the file extension is just passed on to the
+    // MIME type mapper.
+    case RESPONSIVE_IMAGE_ORIGINAL_IMAGE:
+      break;
+
+    // In all other cases, get the file extension of the derivative image from
+    // the image style configuration.
+    default:
+      $extension = ImageStyle::load($image_style_name)->getDerivativeExtension($extension);
+      break;
+
   }
   return \Drupal::service('file.mime_type.mapper')->getMimeTypeForExtension($extension);
 }