I have a similar problem. My attachments in emails fail to open (Godaddy Webmail). Using Firefox they open automatically in a different window. With Brave a window opens, but replaces the email window I am viewing and shows a red line with the words "An error occurred while loading the PDF" the more information box gives the following: “PDF.js v2.0.673 (build: 31012570) Message: Failed to fetch”. When I click to go back to the email I was viewing, it has closed and returned to inbox page. The browser is of no use to me if I cannot open the attachments in my emails. Tried what you have mentioned above, shields off. Also tried moving the file to a new tab, but it just opens my inbox email page again. Also tried on two different laptops, Always same problem.