custom file field not working well with relationship api using webform

Using CiviCRM Webform

1. Create Custom field for relationship type
2. Using webform upload file, webform first it upload to drupal file directory and then upload file using civicrm file api.
3. When relationship API call initiated in web-form custom field contain the file id,
but CRM_Core_BAO_CustomField::formatCustomField() need $value need be an array with file information but it passes file id ( for data_type = file type)