factories.rb

Factory.define :application do |factory|
  factory.attachment(:sample, "public/samples/sample.doc", "application/msword")
end

factory_attachment.rb

require 'action_controller/test_process'
Factory.class_eval do
  def attachment(name, path, content_type = nil)
    full_path = File.expand_path(path,RAILS_ROOT)
    uploaded_file = ActionController::TestUploadedFile.new(full_path, content_type)
    add_attribute name, uploaded_file
  end
end