Hello,

Regrettably, this is currently not supported, as the Forums module uses a completely different form control that is designed to upload files only in the Forums. If you want to allow your users to upload more than one file, you will need create more fields of Field type. You can, however, create a custom form control so that users can upload more files. In that case, you will need to implement saving files using custom code: Developing form controls

Best regards,
Michal Legen