should explicitly returns nil in else clause

actionview/lib/action_view/helpers/asset_url_helper.rb

@@ -237,7 +237,11 @@ module ActionView
       def compute_asset_extname(source, options = {})
         return if options[:extname] == false
         extname = options[:extname] || ASSET_EXTENSIONS[options[:type]]
-        extname if extname && File.extname(source) != extname
+        if extname && File.extname(source) != extname
+          extname
+        else
+          nil
+        end
       end
 
       # Maps asset types to public directory.