A user was experiencing this issue. Forwarding the email back to the user and another user showed the attachment fine.
The attachment showed on mobile devices/webmail and also via a third party Outlook Attachment Add in
Looking at the headers of the email the Mime type was multipart/alternative this should be encoded to multipart/mixed
multipart/alternative is for sending text/html and text/plain versions of a message, and multipart/mixed is for adding attachments.
1) Speak to user sending mail to find out why emails are being sent as multipart/alternative
2) Create a hub transport rule to re-write multipart/alternative to multipart/mixed