Looking at html code generated, I would say that the additional fields used on this forum are added using acf_form() https://www.advancedcustomfields.com/resources/acf_form/ with 'form' => false. This outputs the ACF fields without a wrapping form tag.
The add attachment is a repeater and the button for “Add Row” has been customized using CSS. The repeater for the file attachment has been inserted between the BBP reply field and the BBP form actions field.
To get more information about it your best bet would be to submit a support ticket https://support.advancedcustomfields.com/new-ticket/ and the developer might respond to it. That’s the best way to get his attention.
