AIUI Joplin effectively does that; its synchronization abilities boil down to "Integrate with file synchronization tools that don't natively create sync folders on your phone so you don't have to pay for a pro Dropbox account".
I just set up a test sync, and the files in my Dropbox Apps/Joplin folder are just Markdown and JSON (there appear to be some dotfiles, but I'll bet they just cache the database graph etc).