I add the same file attachment field to several content types on my site. For each, I have the 'Enable Display field' checkbox checked, but the 'Files displayed by default' remains un-checked.
However, when I upload a file using the node's file uploader widget, the newly uploaded file has the 'display' checkbox checked on the node edit form.
Please contact me using this site's contact form (or toddgee.com) if you want more info for this defect. thanks!