factories: add traits about files

spec/factories/dossier_operation_log.rb

@@ -1,5 +1,9 @@
 FactoryBot.define do
   factory :dossier_operation_log do
     operation { :passer_en_instruction }
+
+    trait :with_serialized do
+      serialized { Rack::Test::UploadedFile.new('spec/fixtures/files/white.png', 'image/png') }
+    end
   end
 end