diff --git a/system/src/Grav/Common/Page/Medium/ImageMedium.php b/system/src/Grav/Common/Page/Medium/ImageMedium.php
index b250e4da0..3f35633c8 100644
--- a/system/src/Grav/Common/Page/Medium/ImageMedium.php
+++ b/system/src/Grav/Common/Page/Medium/ImageMedium.php
@@ -590,7 +590,8 @@ class ImageMedium extends Medium
             return $this->result;
         }
 
-        $this->format($this->get('extension'));
+        $extension = strtolower($this->get('extension'));
+        $this->format($extension);
 
         if (!$this->debug_watermarked && $this->get('debug')) {
             $ratio = $this->get('ratio');