We have been digging to find out how to perform tailoring on making attachment mandatory. And as it turns out, the attachment 'function' is part of form designer and there's is no simple way to make the attachment mandatory in comparison with other fields except with MF internal engineering assistance. There is already an internal ER since 2016. [ as attached ] And subsequent to this ER, the link below also suggest the same.
There should be consistency in product feature in that while other fields can be made mandatory through tailoring but one cannot enforce mandatory attachment.