Cannot open attachment in thread in Mail

I have a lengthy thread in Mail which includes an attachment which was sent to someone who forwarded the email-with-attachment to me, then other email exchanges followed.

I cannot open the attachment from the thread (although I could open it initially when I first received the email).

I have tried Preferences, Viewing and unchecking "include related messages" as has been previously recommended, but it doesn't work.

Any ideas pls? Thanks