title: How to send arguments into Paperclip processor
body: <p>i'm trying to understand how to send a model value into a Paperclip custom processor, and just can't figure out why it is so difficult, or what the solution might be, since i'm trying to solve this for some days now...
  Here is my code, extracted from both my model and processor.</p>
  
  <p>From my model :</p>
  
  <pre><code>...
    has_attached_file :receipt_file,
                      :storage =&gt; :s3,
                      :s3_credentials =&gt; "#{Rails.root}/config/s3.yml",
                      :path =&gt; "/:style/:id/:filename",
                      :s3_protocol =&gt; "https",
                      :styles =&gt; { :text =&gt; { style: :original, receipt_id: self.id }},
                      processors: [:LearnProcessor]
  ...
  </code></pre>
  
  <p>Why can't I use the "self.id" to get the receipt Id ? 
  How is it that <code>"/:style/:id/:filename"</code> gets translated into something like <code>/original/1/abc.pdf</code>, and if I put <code>receipt_id: :id</code>, all i get from <code>options[:receipt_id]</code> (see below) is <code>:id</code> instead of <code>1</code> ?</p>
  
  <p>Will I need some kind of Interpolation ?</p>
  
  <p>Processor Code</p>
  
  <pre><code>module Paperclip
  
      class LearnProcessor &lt; Processor
        attr_accessor :receipt_id,:style
  
  
        def initialize(file, options = {}, attachment = nil)
          @file           = file
          @current_format = File.extname(@file.path)
          @basename       = File.basename(@file.path, @current_format)
          @style = options[:style]
          @receipt_id = options[:receipt_id]
          puts "Options #{options.inspect}"
        end
  ...
  </code></pre>