attachment_fu save thumbs as png

Question

How do I save a thumbnail as a png using attachment_fu?

The code below saves them as a jpeg.

  has_attachment :content_type => :image, 
                 :storage => :s3,
                 :max_size => 6000.kilobytes,
                 :thumbnails => { :thumb => 'crop: 160x112'}  

Thanks!!

Deb