diff --git a/core/modules/responsive_image/responsive_image.module b/core/modules/responsive_image/responsive_image.module
index 2fa9761..03989dc 100644
--- a/core/modules/responsive_image/responsive_image.module
+++ b/core/modules/responsive_image/responsive_image.module
@@ -191,7 +191,7 @@ function theme_responsive_image($variables) {
   $sources = array();
 
   $image = \Drupal::service('image.factory')->get($variables['uri']);
-  $mime_type = $image->getMimeType();
+  $extension = pathinfo($image->getSource(), PATHINFO_EXTENSION);
   $responsive_image_mapping = ResponsiveImageMapping::load($variables['mapping_id']);
   // All breakpoints and multipliers.
   $breakpoints = \Drupal::service('breakpoint.manager')->getBreakpointsByGroup($responsive_image_mapping->getBreakpointGroup());
@@ -210,7 +210,7 @@ function theme_responsive_image($variables) {
               // Get the dimensions.
               $dimensions = responsive_image_get_image_dimensions($image_style_name, array('width' => $variables['width'], 'height' => $variables['height']));
               // Get MIME type.
-              $derivative_mime_type = responsive_image_get_mime_type($image_style_name, $mime_type);
+              $derivative_mime_type = responsive_image_get_mime_type($image_style_name, $extension);
               $derivative_mime_types[] = $derivative_mime_type;
 
               // Add the image source with its width descriptor. When a width
@@ -228,7 +228,7 @@ function theme_responsive_image($variables) {
             break;
           case 'image_style':
             // Get MIME type.
-            $derivative_mime_type = responsive_image_get_mime_type($mapping_definition['image_style'], $mime_type);
+            $derivative_mime_type = responsive_image_get_mime_type($mapping_definition['image_style'], $extension);
             $derivative_mime_types[] = $derivative_mime_type;
             // Add the image source with its multiplier.
             $srcset[floatval(drupal_substr($multiplier, 0, -1))] = array(
@@ -320,9 +320,9 @@ function theme_responsive_image_source($variables) {
 /**
  * Determines the dimensions of an image.
  *
- * @param $image_style_name
+ * @param string $image_style_name
  *   The name of the style to be used to alter the original image.
- * @param $dimensions
+ * @param array $dimensions
  *   An associative array containing:
  *   - width: The width of the source image (if known).
  *   - height: The height of the source image (if known).
@@ -347,22 +347,24 @@ function responsive_image_get_image_dimensions($image_style_name, $dimensions) {
 }
 
 /**
-* Determines the MIME type of an image.
+ * Determines the MIME type of an image.
  *
- * @param $image_style_name
+ * @param string $image_style_name
  *   The image style that will be applied to the image.
- * @param $mime_type
- *   The original MIME type of the image.
+ * @param string $extension
+ *   The original extension of the image (without the leading dot).
  *
  * @return string
  *   The MIME type of the image after the image style is applied.
  */
-function responsive_image_get_mime_type($image_style_name, $mime_type) {
+function responsive_image_get_mime_type($image_style_name, $extension) {
   if ($image_style_name == RESPONSIVE_IMAGE_EMPTY_IMAGE) {
     return 'image/gif';
   }
-  ImageStyle::load($image_style_name)->getDerivativeMimeType($mime_type);
-  return $mime_type;
+  // 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);
 }
 
 /**
@@ -376,4 +378,3 @@ function _responsive_image_image_style_url($style_name, $path) {
   }
   return ImageStyle::load($style_name)->buildUrl($path);
 }
-