app/views/picture_comprehension_exercises/_form.html.erb
<%= form_for @picture_comprehension_exercise do |f| %>
<%= f.label :name %>
<%= f.text_field :name %>
<fieldset>
<%= f.label :images %>
<%= f.file_field :images, multiple: true, name: '[images][]' %>
</fieldset>
<%= f.submit "Save", class: "btn btn-primary" %>
<% end %>