Chatty used to use a libpurple plugin, but the dev realized the issues that came with it (as the old comment outlined), and it was moved into Chatty proper, which also allowed for a much cleaner SMS/MMS implementation too.
> MMS attachments (images, didn't try video), group chats, everything was fine.
MMS on Chatty actually supports arbietrary attachments, so you could send a binary file, a PDF, Video, whatever. Android and iOS will not understand them, but you can send it to another Librem 5 or Pinephone. One neat thing I tried was to use GPG over MMS. It was successful, though very manually intensive. There was thought into making GPG over MMS transparent in Chatty: https://source.puri.sm/Librem5/chatty/-/issues/671