title: paperclip - converting tiff images into jpg or png
body: <p>I'm using paperclip to allow user to upload images and I want to convert an image into jpg/png
  only if it's a tiff image. </p>
  
  <p>I am using the following code in my image.rb:</p>
  
  <pre><code>validates_attachment_presence :data
    validates_attachment_content_type :data,
    :content_type =&gt; ['image/jpeg', 'image/pjpeg',
                                     'image/jpg', 'image/png', 'image/tif', 'image/gif'], :message =&gt; "has to be in a proper format"
  </code></pre>
  
  <p>I would like to know if it is possible to convert tiff images into jpg or png when uploading them.</p>
  
  <p>Thanks a lot</p>