If you sign-in to the Dynamics 365 portal as an administrator and create a new child file with a .js file as the attachment it will seem that it will work as expected but when you browse to that new web file URL you will be presented with a blank screen. If you look at your browser tools you may notice that the portal actually returned a HTTP 404 (File Not Found) error. But in this case it does not redirect you to the standard Page Not Found portal page. So what happened? The portal seems to know the URL (or route) of the file I uploaded but isn’t showing any of the content. If you navigate to the back-end CRM and go to the Portals > Web Files entity, you will notice your Web File is infact there, but when you open it you may notice its missing your Note record with an attachment.
Web Files utilize notes (annotations) to store the actual file that will be surfaced with the Web File. If you try now creating a note with your attachment the issue will be revealed that the attachment was blocked due to “the attachment is not a valid file type”.