[Rails 3.2.5] Rails: unit test fixture_path : fixture_file_upload cannot find the file ...

I am trying to perform a test unit, using FactoryGirl

include ActionDispatch::TestProcess
FactoryGirl.define do
  factory :article do
    photo { fixture_file_upload '/files/test.jpg', 'image/jpg' }

IN mt test_helper.rb I defined the fixture_path

  def fixture_path
    File.dirname(__FILE__) + "/fixtures/" # "..../myapp/test/

and I have my test.jpg in "..../myapp/test/fixtures/files/

but running the unit test, article = FactoryGirl.build(:article,
title: nil), I get an error :
RuntimeError: files/test.jpg file does not exist

where should I put the file for 'fixture_file_upload' can find it ?

thanks for feedback

I looked into : actionpack/lib/action_dispatch/testing/

    def fixture_file_upload(path, mime_type = nil, binary = false)
      fixture_path = self.class.fixture_path if self.class.respond_to?
mime_type, binary)

it's trying to get the fixturePath from self.class ( Article in this
case) which doesn't respond to fixture_path
however the correct path is given by

so using the trick:
     photo { fixture_file_upload
"#{ActionController::TestCase.fixture_path}files/test.jpg", 'image/
jpg' }
runs well... but I guess it's just a trick, the 'truth' is somewhere
else ..... ?
how can I relate the fixture_path with the self.class

thanks for your feedback