Refactor message attachements to use active_storage

spec/factories/commentaire.rb

@@ -7,5 +7,9 @@ FactoryBot.define do
         commentaire.dossier = create :dossier, :en_construction
       end
     end
+
+    trait :with_file do
+      file { Rack::Test::UploadedFile.new("./spec/fixtures/files/logo_test_procedure.png", 'application/pdf') }
+    end
   end
 end