title: Paperclip - Validate File Type but not Presence
body: <p>I am using paperclip to handle my file uploads, and in one situation I don't want the file to be mandatory. I do however want to make sure it is a specific file type when it is present.</p>
  
  <p>I have this:</p>
  
  <pre><code>class TestModel &lt; ActiveRecord::Base
      #stuff
      has_attached_file :sound #etc...
      validates_attachment_content_type :sound, :content_type =&gt; ['audio/mp3', 'application/x-mp3']
  end
  </code></pre>
  
  <p>And when I have no sound file present, it tells me it is not one of the valid content types. I've tried adding <code>''</code> to the <code>:content_type</code> array, which also doesn't work!</p>
  
  <p>I also attempted creating a lambda procedure for the <code>:if</code> property, but I can't get it to run without some kind of error.</p>
  
  <p>Anything missing here?</p>