Error in handling API result in Views File field plugin

The commit https://git.drupalcode.org/project/cmrf_core/-/commit/0c208c020d8f229f9a... introduced an error when $views_data[$this->realField]['cmrf_original_definition']['file_api.is_single'] is set to TRUE. This flag isn't checked, anymore.

If a check of the flag would be added to the existing if it would become complex and even more difficult to understand. Maybe the checks should be done in an additional method that name makes clear what is actually checked.