How to assign attachment to document record

Summary:

Managers are submitting termination and including resignation letter in the attachment. Need to have the attachment accessible in document records for HR admins.

Content (please ensure you mask any confidential information):

I had configure the document type but how do I link Termination's attachment to that document type?

Version (include the version you are using, if applicable):

24A

Code Snippet (add any code snippets that support your topic, if applicable):