Hi @wenbin,
The checks to see whether an author can upload a file are implemented in the mayEditPaper function: https://github.com/pkp/ocs/blob/master/classes/submission/author/AuthorAction.inc.php#L293
If the most recent decision on the paper is “revisions required”, but you still can’t upload the file, I suspect that your submission is either archived or declined.
Regards,
Alec Smecher
Public Knowledge Project Team