Currently it is not supported to get the attachments from an attached e-mail.
The triggers on a shared mailbox are very limited. Sometimes it is favorable that a human could manually trigger a flow upon a specific e-mail. Typically e-mails that come in to many size, shapes, from too many domain, too many addresses, etc. to be filtered out automatically.
By forwarding an e-mail with the original e-mail attached on which some flow must run, we could have create this "manual trigger", and add some extra parameters in the subject and body. From the original e-mail we could then grab the attachments, the sender, the Cc, the Bcc, the ReplyTo, the body, the original .EML itself,... further on in the flow.