yea this is likely a configuration problem with your forum, perhaps to do with CDN or S3.
i could be wrong, but i believe it is how uploads are being served on your forum, whether it’s local storage or S3 /object/ CDN. it could be the HTTP response header?
i think if the content-dispostion: attachment header is present, browsers should usually download the file, but if it is missing, browsers often open .txt / .yml.
test.yml (5 Bytes)
^ that .yml file should download as expected.
edit: i think this topic may be relevant: