The file uploads will be saved to the Forms database before submit the form in order to display the uploaded files on the form. If you close the form without submitting it, the files will become unreferenced by any form submissions/drafts and there won't be any instance generated for such file upload(Instance will only be generated with form submission)
Currently the data maintenance won't clean up such files and we have a backlog item to support that. You can run "delete from [dbo].[cf_bp_attachment_data] where attachment_id not in (select [attachment_id] from [cf_bp_data_attachment_mapping])"(make sure only to execute this only when there is no user filling forms as it will delete files for the users that are currently filling the form and not submit yet) on the Forms database directly to clean up such data.