factories.rb

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

factory_attachment.rb

# Support for Paperclip factories (add this before you load your factory definitions)
include ActionDispatch::TestProcess

Factory.class_eval do
  # Default content type is +image/jpg+
  def attachment(name, path, content_type = "image/jpg")
    path_with_rails_root = Rails.root.join(path)
    uploaded_file = fixture_file_upload(path_with_rails_root, content_type)

    add_attribute name, uploaded_file
  end
end