I've just set up mu4e, and have noticed that when opening an attachment with the o key it will open with MuPDF. Pressing A w to open-with doesen't seem to help either, since it prompts for an external shell command.
I'd like to open PDF attachments with pdf-tools instead of MuPDF, or at least be able to open with with the open-with function mapped to A w
Ae, for "in-emacs", opens the attachment in PDFView. Have you tried that? Also,oeworks exactly the same.