If you've copied the file locally to a Windows system, then you'd have to use a Windows path:
If it's online, it will externally always use UNIX-style paths:
Even if the server is IIS, if you are trying to reach the file externally there will be UNIX-style urls. This is why even though there are plenty of IIS servers out there on the web, all web URLs are still domain.com/path/to/file
Internally from a Windows machine to local IIS server, I have no idea, but I'd think it would be 100% Windows-style paths (so no file:/// stuff).