How to make Rails's four directories "fonts, images, scripts and stylesheets" displayed in Safari's page resources?

Rails version: 3.2.21

When I run a project vish, when accessing "http://localhost:3000" and open Safari's "show page source" there are four directories fonts, images, scripts and stylesheet in the left-side as following picture viewed:

When I built my own rails app project based on the same platform, there is no directories except files under localhost:

What should I do for such effect? What options should I config?