rails-docstore/app/views/documents/_form.html.erb

<%= form_for document, url: documents_path, html: { multipart: true } do |f| %>
  <% if document.errors.any? %>
    <div id="error_explanation">
      <h2><%= pluralize(document.errors.count, "error") %> prohibited this document from being saved:</h2>

      <ul>
      <% document.errors.full_messages.each do |message| %>
        <li><%= message %></li>
      <% end %>
      </ul>
    </div>
  <% end %>

  <div class="field">
    <%= f.label :doc %>
    <%= f.file_field :doc %>
  </div>

  <div class="actions">
    <%= f.submit %>
  </div>
<% end %>