Factory with carrierwave upload field

Hello i need to build up Factory for my model, for example

Factory.define :farm do |f|
  f.name { Factory.next :name }
  f.harvest '3'
  f.offers 'Random'
  f.latitude '43'
  f.longitude '-70'
  f.about 'We rocks!'
  f.logo { Factory.next :logo } # this doesn't work
end

For now im just pass string "#{n}.jpg" into my logo field and this dont work, how to evalute this field? Im using CarrierWave for uploading.