Migrating Commerce File Field with Multiple Source Data Row

Has anyone tried migrating commerce file fields (https://www.drupal.org/project/commerce_file) with multiple source data rows?
I made it worked to some fields except commerce_file fields, it only migrated 1 id where it should have multiple id's.
I've checked the query in the source database, it outputs correctly, my problem is upon migration.

Here's what I did.

$query->addExpression('GROUP_CONCAT(file.field_file_fid)', 'file_id');
.
.
.
.
$this->addFieldMapping('commerce_file', 'file_id')
        ->sourceMigration('PrivateFile')
        ->separator(',');
$this->addFieldMapping('commerce_file:file_class')
        ->defaultValue('MigrateFileFid');
$this->addFieldMapping('commerce_file:preserve_files')
        ->defaultValue(1);

Thanks for any help!