I can confirm that files and uplodas would work in composer when: allow uncategorized topics is disbaled and no category for the topic is selected the file would be uploaded and save on either drag and drop or browse.
But I am not sure what exactly is the issue, since the topic is still draft, the expected behavouir might not be straight forward, regarding this:
How would that hold with save draft feature?
Also interestingly when the draft is deleted the file would still exist in the uplodas/defualts, would those orphaned files be deleted eventually? I am not sure what is the answer, but releavnt long discussion can be found at Is there a way to delete old/irrelevant uploads?