There is a problem with forms that have the "Allow Users to Edit Completed Submission" option activated and contain a required "File Upload" field. This problem occurs when the user who submitted the form wants to edit any field other than the required "File Upload" field and re-submit the form. If the user updates a field other than the required "File Upload" field and chooses not to remove an already uploaded file or to upload another file, because the file already uploaded was correct, they will be unable to submit the form because they will get a warning that the "File Upload" field is required with the message: "This field is required. Please upload a file."
Currently, the work around for this is to make the "File Upload" field not required, but this isn't really an acceptable solution. I have reported this bug to Appnitro.