How to have a document type appear in the attachment LOV for a Journey?

Summary: Are there any specific flags on the document type, or fields within the Journey, that need to be configured a specific way in order for an item to be added to the list of values under "Add attachments for document records"?

Content (please ensure you mask any confidential information):

Hillsborough County requests that employees save a copy of their benefits confirmation statement after enrollments and it must be presented in cases of discrepancy. With Redwood Benefits eliminating the confirmation screen after election submission, we are creating an journey task to help remind employees to perform this task. We have created a unique document type called "Benefits Confirmation Statement" under the document category benefits, however, we are unable to select it (or any other document type) under "Add attachments for document records." Are there any specific flags on the document type, or fields within the Journey,