Nearly every time when the users reply to a thread, they get an error and click on reply again, resulting in a double post. I see this in the logs:
InvalidArgumentException: DOMXPath::query(): Invalid expression in selector >> body << in...
I am looking into migrating to Xenforo from SMF also, after using SMF for almost 10 years. Try this add-on; I think that it will do what you need (display attachments full-size):
https://xenforo.com/community/resources/insert-attachment.2544/