We have an alternate solution to adding attachments after a sales order has been closed and released.
This is done by uploading attachments to the invoice on the closed sales order. We need to get to the correct location on the "Document Flow" section. We get there by opening the sales order and selecting first the "View All" button at the top right-hand corner of the screen to open up all of the options. The "Document Flow" section shows the invoices for the sales order near the bottom. We select any invoice from that sales order by selecting the link on the invoice. Then we select "View All" button at the top right-hand corner of the screen to open up all of the options. We then have to go below under the General tab link to the attachments section, and we can add files there by browsing for the correct files and uploading those attachments to the closed sales order in this way. Unfortunately, we cannot add attachments directly to a closed sales order without starting that sales order completely over again or creating a new sales order.
We can continue to attach files as needed to these invoices as described above in this way for closed sales orders.