factory_girl_ext.rb

require 'action_controller/test_process' 
class Factory
  module FixtureFileUpload
    def fixture_file_upload(attr_name, path, mime_type=nil, binary=false)
      uploaded_file = ActionController::TestUploadedFile.new(
        Test::Unit::TestCase.respond_to?(:fixture_path) ? Test::Unit::TestCase.fixture_path + path : path, 
        mime_type,
        binary
      )
      add_attribute attr_name, uploaded_file
    end
  end
end

Factory.send(:include, Factory::FixtureFileUpload)