Wow, what a horrible design for maintenance.
Thanks for finding it.
I'm not interested in editing the DB.
As long as I find where the files are, I can clean those up without removing the messages and references.
Looks like I found just that at /app/data/files/.
A visit via the new File Browser in the Cloudron App Config/Console easily finds the few large attachments.
Another thing I noticed is that MM generates preview and thumbnail images for some reason, which more than doubles the storage requirements for each image uploaded. Some of the generated preview images are larger than the original uploads