如何使用Remotipart 1.0和Rails 3.2来保存文件上传

I have this form

我有这个表格

<%= form_for @post, :html => {:multipart => true , :class => 'form-horizontal' }, :remote => true do |f| %>
    <%= f.text_field :title%>
    <%= f.file_field :avatar%>
    <%= f.submit nil%>
<% end %>
<%= form_for @post,