Cacheable Rack::File

file_handler = Rack::File.new('.')
#...
proc { |env|
  if !env['HTTP_IF_MODIFIED_SINCE'] || File.mtime(env['PATH_INFO'].sub(/\//, '')) > Time.rfc2822(env['HTTP_IF_MODIFIED_SINCE'])
    r = file_handler.call(env)
    r[1]['Expires'] = (Time.now + 60*60*24).httpdate if env['PATH_INFO'] =~ /(script|style)/
    r
  else
    [304, {}, ['']]
  end
}