rewrite_extension?

# File actionpack/lib/action_view/helpers/asset_tag_helper.rb, line 724
        def rewrite_extension?(source, dir, ext)
          source_ext = File.extname(source)[1..-1]
          ext && (source_ext.blank? || (ext != source_ext && File.exist?(File.join(config.assets_dir, dir, "#{source}.#{ext}"))))
        end