rewrite_extension

# File actionpack/lib/sprockets/helpers/rails_helper.rb, line 157
        def rewrite_extension(source, dir, ext)
          source_ext = File.extname(source)
          if ext && source_ext != ".#{ext}"
            if !source_ext.empty? && (asset = asset_environment[source]) &&
                  asset.pathname.to_s =~ /#{source}\Z/
              source
            else
              "#{source}.#{ext}"
            end
          else
            source
          end
        end