And I forgot the view code:
<%= simple_form_for(@candidate) do |f| %>
<%= f.error_notification %>
<div class="form-inputs">
<%= f.input :first_name %>
<%= f.input :last_name %>
<%= f.simple_fields_for :documents, @documents.to_a do |d| %>
<div class="file-field">
<%= d.input :file, label: d.object.label, label_html: {class: 'file'} %>
<%= d.check_box :_delete, label: 'Remove', label_html: {class: 'delete check_box'} %>
<%= d.hint d.object.file_name.to_s %>
<%= d.hidden_field :style, class: 'style' %>
<%= d.hidden_field :id, class: 'id' %>
</div>
<p class="add"><a class="btn">Add</a></p>
<%- end -%>
</div>
<div class="form-actions">
<%= f.button :submit %>
</div>
<% end %>