diff --git a/lib/linguist/blob_helper.rb b/lib/linguist/blob_helper.rb
index aa55984b..85080b2b 100644
--- a/lib/linguist/blob_helper.rb
+++ b/lib/linguist/blob_helper.rb
@@ -148,7 +148,7 @@ module Linguist
     #
     # Return true or false
     def image?
-      ['.png', '.jpg', '.jpeg', '.gif'].include?(extname)
+      ['.png', '.jpg', '.jpeg', '.gif'].include?(extname.downcase)
     end
 
     # Public: Is the blob a supported 3D model format?