class Middleman::Extensions::CacheBuster

def asset_url(path, prefix='')
  http_path = super
  if http_path.include?('://') || !%w(.css .png .jpg .jpeg .svg .svgz .js .gif).include?(File.extname(http_path))
    http_path
  else
    if respond_to?(:http_images_path) && prefix == http_images_path
      prefix = images_dir
    end
    real_path_static = File.join(prefix, path)
    if build?
      real_path_dynamic = File.join(build_dir, prefix, path)
      real_path_dynamic = File.expand_path(real_path_dynamic, root)
      http_path << '?' + File.mtime(real_path_dynamic).strftime('%s') if File.readable?(real_path_dynamic)
    elsif resource = sitemap.find_resource_by_path(real_path_static)
      if !resource.template?
        http_path << '?' + File.mtime(resource.source_file).strftime('%s')
      else
        # It's a template, possible with partials. We can't really
        # know when it's updated, so generate fresh cache buster every
        # time during developement
        http_path << '?' + Time.now.strftime('%s')
      end
    end
    http_path
  end
end