form
...
<%= f.fields_for :pictures do |pic| %>
  <%= render 'picture_fields', f: pic %>
<% end %>
...

response body

<script type="text/javascript">try{window.parent.document;}catch(err){document.domain=
document.domain;}</script>
<textarea data-type="text/javascript" data-status="200" data-statusText="OK">
$("div#id5").html("<form class="new_gallery" id=.... LOTS OF CODE....</form>");</textarea>

form
...
<%= f.fields_for :pictures do |pic| %>
  <%= pic.file_field :picture %>
<% end %>
...

response body

<script type="text/javascript">try{window.parent.document;}catch(err){document.domain=
document.domain;}</script>
<textarea data-type="text/javascript" data-status="200" data-statusText="OK">
$("div#id5").html("<form class="new_gallery" id=... LOTS OF CODE....</form>");</textarea>