To fix attachments on older templates go to post_attachments_new and change input name parameter from "attachment" to "attachments[]". That's how I fixed the issue for my forums.
I had the same problem and what I did was to copy that line from a working theme.
<input type="file" name="attachments[]" size="30" class="form-control" multiple="multiple" />
@hfreire very old code!
Attachment-System changed 2021