title: Displaying file field value on edit
body: <p>When upload pictures, everything goes okay.
  But on edit, it doesn't display the file fields value. Just an empty file_field, like nothing is there. Pic title displays correctly.
  Other text is in hungarian.</p>
  
  <p><img src="https://i.stack.imgur.com/VIVoE.png" alt="enter image description here"></p>
  
  <p>_form.html.haml</p>
  
  <pre><code>= simple_nested_form_for(@post) do |f|
    = f.input :title, label: 'Cím'
    = f.input :body, label: "Test"
    = f.fields_for :pics do |pic_form|
      = pic_form.text_field :title
      %br/
      = pic_form.file_field :image
      = pic_form.link_to_remove "Kép Törlése", class: "btn btn-warning"
      %br/
    %br/
    %p= f.link_to_add "Kép hozzáadása", :pics, class: "btn btn-success"
    %br/
    %br/
    = f.submit "Mentés", class: "btn btn-primary"
  = javascript_include_tag :defaults, "nested_form"
  </code></pre>
  
  <p>How to pass, the existing file to the file_field?</p>