I know this is a really old thread but it helped me. I am using nopCommerce 3.9 in Server 2016 Core. On my Desktop I have Windows 7 (Still), FireFox 67.0.2 and IE 11.0.9600.19356. I also found that I could not upload files using FireFox, although on rare occasions it would, including a PDF, but it was maybe 1 in 100 so not sure what that is about. Anyway I just needed to get stuff done so in reading this I tried IE and found I could successfully upload files. I can view and select them in either browser.