title: Paperclip validates_attachment_content_type for mp3 triggered when attaching mp3
body: <p>Struggling to workout when i add the following validtion to my Voice model using paperclip, it is being triggered when i try and upload an mp3:</p>
  
  <pre><code>class Voice &lt; ActiveRecord::Base
    has_attached_file :clip
  
    validates_attachment_presence :clip
    validates_attachment_content_type :clip, :content_type =&gt; [ 'application/mp3', 'application/x-mp3', 'audio/mpeg', 'audio/mp3' ],
                                      :message =&gt; 'file must be of filetype .mp3'
  
    validates_attachment_size :clip, :less_than =&gt; 10.megabytes                                    
  
    validates_presence_of :title      
  end
  </code></pre>
  
  <p>I have tried a number of different mp3 files but none of them seem to upload because the validation is failing.</p>