Preventing deletion of files on rollback not working

Hi,

I have a serious problem migrating D6 content type with file (video) into my new D7.
read a lot of things but not found any solution. Trying to explain it here, forgive my bad english :)

Because of legacy D6 content type have a video field (video module) ; I have installed video module content in my D7.

First

I make a Files migration class to migrate Fields separately.
This migration class is registered like that :

...
$this->current_args = array(
            'class_name' => 'MyMigrateFile',
            'machine_name' => 'Fichiers',
            'description' => '',
            'default_uid' => 1,
            'source_dir' => 'public://',
            'destination_dir' => 'public://',
            'preserve_files' => true,
            'user_migration' => 'MyUsers',
        );
...
Migration::registerMigration($this->current_args['class_name'], $this->current_args['machine_name'], $this->current_args);
...

Class is :

class MyMigrateFile extends DrupalFile6Migration {

    public function __construct(array $arguments) {
        parent::__construct($arguments);

        $this->addUnmigratedSources(array('origname'));
        $this->addFieldMapping('path')->defaultValue(false);
        $this->addFieldMapping('urlencode')->defaultValue(true);
        $this->addFieldMapping('preserve_files')->defaultValue(true);
    }
...
}

This migration can run / rollback without error, all is ok.

Then

I want to migrate my video based content type so I create my custom class

class MyMigrateNodeVideo extends DrupalNode6Migration {

    public function __construct(array $arguments) {
        parent::__construct($arguments);

        $useless_fields = array(
            ...
        );
        $this->addUnmigratedSources($useless_fields);
...

        $this->addFieldMapping('field_video', 'field_video_contenu')->sourceMigration('Fichiers');
        $this->addFieldMapping('field_video:file_class')->defaultValue('MigrateFileFid');
        $this->addFieldMapping('field_video:preserve_files')->defaultValue(TRUE);
        $this->addFieldMapping('field_video:thumbnail')->description('cf complete()');

        ...
    }
}

This migration run fine, file rows are created in file_usage, and I can see video into my new content.
But when I rollback this migration, file_usage row are deleted too.
At the consequence, when I migrate again, I supposed new file ID is not found by migrate so the file is not "created" or "linked" to the node, and file_usage stay empty

Why "file_preserve" not working in this case ?

Need help, very appreciate :)

Thanks.

Note I use Migrate 2.6 and Migrate D2D together, and dynamics migrations.
Read that https://drupal.org/node/1061560 without success