title: Validate Attachment Content Type Paperclip
body: <p>Is it possible to enforce a 'content type' validation in <a href="http://github.com/thoughtbot/paperclip" rel="noreferrer">paperclip</a> without enforcing a 'presence' validation (i.e. allow blanks)? I currently have:</p>
  
  <pre><code>class Person &lt; ActiveRecord::Base
    has_attached_file :picture
    validates_attachment_content_type :picture, :content_type =&gt; ['image/jpeg', 'image/jpg', 'image/png']
  end
  </code></pre>
  
  <p>However, this fails if no attachment is present. For example:</p>
  
  <pre><code>&gt;&gt; @person = Person.new
  &gt;&gt; @person.save
  &gt;&gt; @person.errors.first
  =&gt; ["picture_content_type", "is not one of image/jpeg, image/jpg, image/png"]
  </code></pre>
  
  <p>Is it possible to do the validation only if an attachment is included.</p>