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)
    path_with_rails_root = "#{RAILS_ROOT}/#{path}"
    uploaded_file = if content_type
                      ActionController::TestUploadedFile.new(path_with_rails_root, content_type)
                    else
                      ActionController::TestUploadedFile.new(path_with_rails_root)
                    end

    add_attribute name, uploaded_file
  end
end