The note custom field allows for documents to be attached. However, when "attachments" are synchronized between Aha! and the development system, only attachments linked to the Description field are sent over. It would be ideal if the documents attached to the custom fields were also sent through as attachments, providing that the custom field is mapped as well.
My integration is with Azure DevOps and have the same need. If you support attachments on custom fields and the ability to map those custom fields to other systems, then their meta data that is allowed (that I cannot disable) should be supported.
In addition, the knowledgebase content should be very clear that ONLY the Description field attachments is supported at this time. It is not feasible to believe that a company is able to invest in additional custom API integration vs those out of the box, given that you allow the meta data and mapping.