factories.rb

FactoryGirl.define do
  factory :application do
    attachment :sample, "public/samples/sample.doc", "application/msword"
  end
end

factory_attachment.rb

require ActionDispatch::TestProcess

module FactoryGirl
 DefinitionProxy.class_eval do
   def attachment(name, path, content_type = nil)
     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
end