title: URL Generation for paperclip document based on version
body: <p>I am trying to implement file versioning in my application. I  have the following model in my application. </p>
  
  <pre><code>    class Document &lt; ActiveRecord::Base
       versioned
       attr_accessible :name,:file,:attachable_type,:attachable_id
       belongs_to :user
       belongs_to :attachable , :polymorphic =&gt; true
       has_attached_file :file, :preserve_files =&gt; true
       Paperclip.interpolates :version do |attachment, style|  
        attachment.instance.version.to_s  
       end
      end
  </code></pre>
  
  <p>Paper clip uploads all documents to Amazon S3.The path is as follows.</p>
  
  <pre><code>    Paperclip::Attachment.default_options[:path] = '/:class/:attachment/:id_partition/:version/:style/:filename'
  </code></pre>
  
  <p>If I use </p>
  
  <pre><code>    document.file.url
  </code></pre>
  
  <p>I am able to get the path to latest file. How to get the url path for a specific version of the file?
  I tried passing version as a argument to the url method, but it doesn't seem to work.</p>
  
  <pre><code>    document.file.url(:original, :version =&gt; 2)
  </code></pre>
  
  <p>Any help is greatly appreciated.</p>