title: Mixed file types with CarrierWave
body: <p>I've a CarrierWave uploader that shall accept a variety of file types. Some are image types (e.g. jpg, png) others are not.</p>
  
  <p>I would like to create a medium version of the uploaded file with </p>
  
  <pre><code>version :medium do
    process :resize_to_fit =&gt; [300, 300]
  end
  </code></pre>
  
  <p>As this only works for image files, how can I distinguish between images and other types and omit the resizing for non-image files?</p>
  
  <p>At the moment CarrierWave tries to process the file regardless of it's type which leads to a MiniMagick processing error if the file is not an image.</p>