As an alternative, I tried converting the auto-made RTF to Markdown, but of course that failed since doing so just made a new file.
But I don’t mind a workaround like that. Is there some way I can attach a non-RTF file so that it appears as another file’s Inspector Attachment? Or for that matter, how can I attach any file to be the Annotations file linked to another file? Or can this only be done by typing inside the Annotations field and letting DevonThink create an RTF?