Document Previewer fails on PDF files:

We have a customer's installation where the Document preview option works for images, but for PDF documents returns:

PDF.js v0.8.1269 (build: 10deadd) Message: Unexpected server response (403) while retrieving PDF "https://my.customers.domain.net/storage/2017/August/week1/3781_Customer-Details-Form_PDF.pdf".

This bug has been confirmed by another on the developers mailing list.