Did I understand correctly:
You have an InfoPath form with an attachment in the form? In this case there is no versioning of the attachment.
If you browse your library where the form is stored in explorer view (the XML files) and open one in editor (notepad) there would be a encoded XML string representing the document.
In this case I don't see a simple solution. If you need to go with this approach you could try to create versions of the document for yourself.
Or do you have a link pointing to a document in your InfoPath form? And you want to access the version history of this doucment?
In this case you might be able to link the SharePoint Version History of the document.