title: Rails paperclip, Edit form file_field not assigned
body: <p>I used paperclip to attach an avatar onto my user, in my Model:</p>
  
  <pre><code>has_attached_file :avatar, 
      :styles =&gt; {square_tiny: '50x50#', square_small: '100x100#', square: '200x200#'}
  </code></pre>
  
  <p>I have a form</p>
  
  <pre><code>&lt;%= form_for(@user_profile, 
          :url =&gt; { :controller =&gt; :user_profiles, :action =&gt; :update_general_info, :id =&gt; @user_profile.id }, 
              :html =&gt; {  :multipart =&gt; true, 
                  :class=&gt; "form-horizontal" }) do |f| %&gt;
  
              &lt;div class="control-group"&gt;
                &lt;%= f.label :avatar, :class =&gt; "control-label" %&gt;
                  &lt;div class="controls"&gt;
                      &lt;%= f.file_field :avatar  %&gt;
                  &lt;/div&gt;
              &lt;/div&gt;
  
  ....
  
  &lt;% end %&gt;
  </code></pre>
  
  <p>The upload works perfect, but I come back and EDIT my user, the file field says 'no file chosen'. And since I am validating presence of that avatar, every time a user edit his details, he has to upload his avatar again... </p>
  
  <p>How do I work around that?</p>
  
  <p>I thought the <code>:multipart =&gt; true</code> would help but it didn't.</p>