Also rewriting the file visualization to work consistently with every
file type. Here is the overall basic behavior now:
  * If the request is passed with view=true, content is displayed
    as an article content.
    * If the file has an inline visualization (like images) it's already
      displayed.
    * If not, a download link is displayed.
  * If the request is passed with view=false, the file is provided
    straight, without any noosfero layout being loaded.
  * If the file is private:
    * And the user accesses its public filesystem path, apache (this is
      done by noosfero-apache) will redirect the request to rails
      path so that the rails server will provide it considering
      appropriate permissions.
    * And the user accesses its rails path, rails server will provide as
      well.
  * If the file is public:
    * And the user accesses its public filesystem path, apache will
      provide the file.
    * And the user accesses its rails path, rails server will redirect
      to its public filesystem path so that apache provides the file.