Review Mozilla 1714583: UNC shared folder as default download location not working on Windows 10 (because nsIFile::exists() returns false for network server names on Windows)

https://bugzilla.mozilla.org/show_bug.cgi?id=1714583

This patch fixes an issue whereby users were unable to save files to a UNC shared folder. We have a few other issues in the past with UNC paths (some unavaoidable iirc) for refernence:

• #26874 (closed)
• #26424 (closed)

We should figure out what the right behaviour is here, maybe revert the above Mozilla fix or more actively prevent UNC path access.