Hi Peter
Unfortunately, this is indeed the behavior of the mail receiver adapter.
Therefore you'd need some custom development to get it to work, i.e. have an email body with attachments.
Following blog shows an example of constructing the MIME content of the email via Java mapping. It does not use Mail Package since Mail Package is deprecated.
Create email with body and attachments for binary payload with Java mapping
However, if you insist on using Mail Package, you can refer to the link within the above blog as well as the solution in the following thread.
How send e-mail with XML + PDF and body text
Regards
Eng Swee