Can't use file protocol to load resources from another IP since 2017.1

In 5.6 and below I've been successfully able to load files at runtime using the file:// protocol with WWW.
Here is an example (this would of course be in a coroutine):
WWW www = new WWW("file://123.456.78.90/test.png");

This does not work in 2017.x, with neither WWW nor UnityWebRequest. I am aware of the WWW deprecation in favour of UnityWebRequest, but haven't been able to make either work in 2017.x.

I have tried
- Both WWW and UnityWebRequest
- file://, file:///, file:////, file:\\
- double slashes instead of slashes in url
- backslashes instead of forward slashes in url
- all of the above on versions: 2017.1.0, .1.1, .1.2, .1.2p2, 2017.2.0p2, 2017.3.0b10

Not once have I been able to get anything but either "Generic/unknown HTTP error" or "404 not found".

- I am developing exclusively on Windows
- I am releasing exclusively on Windows (internal software)
- All my files are hosted on networked Windows machines
- All the files I'm trying to access have no restrictions regarding permissions
- All the files work fine in browsers
- I have tried the machine name as well as the machine IP

This has worked seamlessly in versions prior to 2017.1. I've checked the 2017.1 release notes but can't see anything on this topic.

Was the ability to load files from other IP addresses just somehow silently removed, or am I the one at fault here? Help would be greatly appreciated as I'm kind of stuck on 5.6 until I can resolve this!

 

Thanks for the suggestion, have done!
https://fogbugz.unity3d.com/default.asp?971557_k8l1fr9doimdgteh