Tesseract

Tesseract is a simplistic autoresponse bot with the superpower of being able to read message attachments for text. Simply set up your autoresponses with /response add, and Tesseract will do the rest of processing messages and their attachments, and issuing your responses.

Responses are per-channel and not global.

If required, you can set up a forum's autoresponses by creating a temporary post and then running the required slash commands there. Threads only require the parent channel to have its autoresponses set up, and they'll just work.