A post on macOSXHints reveals a tip 
that was news to me.
If you want to forward selected attachments in an email you have received, highlight them, click Forward (or Command-Shift-F) and a new message appears containing only the attachments you have selected.
Of course, this is just an extension of the standard behaviour with selected text. Blocking a selection of text in an mail and then starting a forwarded message (or a reply for that matter), opens a new message containing only that selected text.
Still, I had never thought of applying it to attachments. Nice one.
This is the tip I have been looking for! It seems odd that forwarding doesn’t include attachments by default, or at least make it an option in the pref pane… I send and receive attachments frequently, and saving them to desktop and then reattaching has been a pain!