How can the reviewer get permission to view uploaded files on the Google Form without having to ask each time?

If the workflow owner is allowing File upload on a Google Form these files will be uploaded to the form owner's Google Drive. The form owner needs to share this folder with reviewers.

To share this folder with reviewers, submit your form once.

Under the RESPONSES tab go to the the question allowing File upload and click on VIEW FOLDER. 

From Google Drive you will be able to Share this folder.