Add fid-based file_function to file handler

If you're using MigrateDestinationFile to import files (as I am at the moment), and you want to attach those files to file fields in later migrations (indeed I do), it would be convenient to just pass the fids mapped from the file migration into the file field:

    $this->addFieldMapping('field_attachments', 'ATTACHMENTID')
         ->sourceMigration('Attachment')
         ->arguments(array('file_function' => 'file_fid'));

Just creating an issue to reference in the commit and CHANGELOG...