Were I you, I would call the folks at Microsoft Windows & Office Accessibility Support on (800) 936-5900 and have them take a look.
ALT+A does not work for me, SHIFT+TAB does. If I customize the read window such that the Attachments tab shows, then ALT+Y will throw me to the controls, but it seems to me you're asking about getting focus on the individual attachments that are attached to the message.
Brian - Windows 10 Pro, 64-Bit, Version 1909, Build 18363
The purpose of education is not to validate ignorance but to overcome it.