If you use the MB Builder, please add the path of the folder. You can create a file test.php in the folder invoices for example and add this code to the file test.php
The file saved in the custom folder won't show on the Media Library. We also note about this case in the documentation.
Unlike the normal case, where files are added to the WordPress Media Library, files uploaded to custom folders are not available in the Media Library. Thus, the data saved in the custom fields is file URL, not attachment ID.