Authors can upload additional supplementary files while a submission is under review by visiting the submission's Summary page, which is fine. 

However, if an author is asked to make revisions to their supplementary file(s) in addition to any manuscript edits, they have to visit two different pages to accomplish these actions: the submission's Summary page for the supplementary file as above; and the submission's Review page for the revised manuscript. 

Moreover, from what I can see there's no decent way to track revised supplementary files that have been uploaded: the Editor will have to cull any supplementary files as they are uploaded from the Summary page. 

I would suggest moving the supplementary file upload to the Review page (not the best solution), or perhaps mirroring the Editor/Author Version manuscript behaviour that can be found under Editor Decision if the journal allows supplementary files to be used in the review process.