[Feature Request] Document Sub-Type

Description

Re-opening #4057

I currently use storage paths like:
Person/{created_year}/Banking/{created_year}-{created_month}-{created_day} [{correspondent}] {document_type} - {title}. The document type could be "Bill" or "Statement".

I'd ideally like to have a document subtype field, so I could do
Person/{created_year}/{document_type}/{created_year}-{created_month}-{created_day} [{correspondent}] {document_sub_type} - {title}
This way I could make the document type "Investments" or "Banking" or "Bills" and could have the sub-type something like "monthly statement" "confirmation" "tax document"

Other

Yes I can do this with custom fields, but it seems common enough to need a second level type.