You would need to check XenForo_Model_Attachment for an example of how things work right now. Also it's not specifically relating to the swfupload feature (bearing in mind there's also a standard file upload button).
You'd also need to create a custom attachment handler with additional content types set up in the database.
It's a pretty long process actually, but basically you need to analyze every single part of the process e.g. the templates included, the attachment controller, the attachment model, the existing attachment handler and the existing content types in the XenForo xf_content_type table and all of that will hopefully give you enough clues on how to proceed.