The issue you are experiencing is part of the issue where values do not get stored in the variables for read-only fields. When a file is uploaded there's a quick page refresh. When that happens and the filled field is read-only, Forms does not store a value to that field on submission.

You could use JavaScript to work around this issue. The following code assumes a filled field has the ro class, and the file upload field has the upload class.

$(document).ready(function(){
  $('.ro input').attr('readonly','True');
  $('.upload input').click(function () {   
    $('.ro input').removeAttr('readonly');
  });
  $('.upload input').blur(function () {
    $('.ro input').attr('readonly','True');
  });
});

So, when the user clicks the file upload button, we remove the read-only attribute from the filled field. That way, when the page refreshes, the field does not have the read-only attribute and Forms will store a value there. We make the field read-only again once the user browses away from the file upload field, which happens when they click the submit button or anywhere else on the form.

With that said, if it is possible, it's probably worthwhile to wait for the maintenance release to fix this issue.

UPDATE: Here's the hotfix that addresses this issue.