rewrite_extension

# File actionpack/lib/sprockets/helpers/rails_helper.rb, line 159
        def rewrite_extension(source, dir, ext)
          source_ext = File.extname(source)[1..-1]

          if !ext || ext == source_ext
            source
          elsif source_ext.blank?
            "#{source}.#{ext}"
          elsif exact_match_present?(source)
            source
          else
            "#{source}.#{ext}"
          end
        end