title: CarrierWave not saving upload after form redisplay
body: <p>I have a rails app which originally used Paperclip for file uploads, however, as I saw that CarrierWave apparently had a 'persistant upload' type feature over form redisplays, I decided to give that a try.</p>
  
  <p>In my view I have the following:</p>
  
  <pre><code>= f.input :attachment
  = f.hidden_field :attachment_cache
  </code></pre>
  
  <p>Which correctly caches the file if the form fails validation and is redisplayed, however when I correct the validation errors and resubmit, the attachment is not processed.</p>
  
  <pre><code>Started POST "/section/model" for 127.0.0.1 at 2012-03-20 08:51:56 +0000
    Processing by Client::WishesController#create as HTML
    Parameters: {"utf8"=&gt;"✓", "authenticity_token"=&gt;"zkdPQBsAnsNzx555rkwZ5+clfnLaXg+NtL+GdFei188=", "model"=&gt;{"title"=&gt;"Sample", "content"=&gt;"Sample content", "contact_name"=&gt;"Mr Sample", "contact_email"=&gt;"sample@example.com", "attachment_cache"=&gt;"20120320-0851-42559-1644/SampleAttachment.pdf"}, "commit"=&gt;"Create Wish"}
    Client Load (0.3ms)  SELECT `clients`.* FROM `clients` WHERE `clients`.`id` = 1 LIMIT 1
     (0.2ms)  BEGIN
    SQL (0.4ms)  INSERT INTO `models` (`attachment`, `client_id`, `contact_email`, `contact_name`, `content`, `created_at`, `status`, `title`, `updated_at`, `upload_content_type`, `upload_file_name`, `upload_file_size`, `upload_updated_at`) VALUES (NULL, 1, 'sample@example.com', 'Mr Sample', 'Sample content', '2012-03-20 08:51:56', 'Unresolved', 'Sample', '2012-03-20 08:51:56', NULL, NULL, NULL, NULL)
  </code></pre>
  
  <p>It appears to be correctly passing the attachment via the attachment_cache in the params, but it isn't then saving the attachment as it doesn't have a params[:model][:attachment] field.</p>
  
  <p>It doesn't say any further steps on the carrierwave section for making uploads work across form redisplays on  the <a href="https://github.com/jnicklas/carrierwave" rel="noreferrer">CarrierWave GitHub</a>.</p>