A user had this setting enabled, Open files directly (using WebDAV) and it was working in 10.3.1 but after upgrading to 10.4.3 it was no longer working, electronic files couldn't be opened using WebDAV. Maybe because 10.4.3 is putting more Windows Auth requirements on IIS websites.
The option isn't even available for me using the Admin account, how did a generic user get the chance to enable it?