Rails 3: Add #attachment FactoryGirl method for defining paperclip attachments on models.

factories.rb

require 'action_dispatch/testing/test_process'

Factory::DefinitionProxy.class_eval do
  def attachment(name, path, content_type = nil)
    path_with_rails_root = "#{Rails.root}/#{path}"
    uploaded_file = if content_type
      Rack::Test::UploadedFile.new(path_with_rails_root, content_type)
    else
      Rack::Test::UploadedFile.new(path_with_rails_root)
    end

    add_attribute name, uploaded_file
  end
end

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

Take from the following gist and updated for Rails 3 comptability:
https://gist.github.com/162881