Rails. Simple form multipart file not working

Question

I use simple_form and multipart uploading does not work(uploading a single file works). Rails file_field_tag work...
Rails 3.2.13

<%= simple_form_for @building, input_html: {multipart: true} do |f| %>
...

  <%= f.simple_fields_for :paintings do |p| %>
    <%= p.input :image, as: :file %>
  <% end %>

<% end %>

Solution

You should use html option instead of input_html:

<%= simple_form_for @building, html: {multipart: true} do |f| %>