All attachments uploads are handled by the WordPress Media Library, and plugin uses native WordPress upload functions. When file is uploaded it will be available through Media Library.
Consult WordPress documentation about Media Library requirements.
Plugin stores all the errors as metadata, and you can see all the errors in the Errors Log panel on the admin side under bbPress Toolbox menu or in the admin side topics and replies editors.
To learn more about errors log, check out this article.