[KevinH]

Well the KDE Plasma file dialog only ruled out non-file scheme urls so it might let a file: url pass through.

Sigil code was generally not designed to accept a url from open file dialogs. It wants file paths or in this case a list of paths not a list of urls.

So I say we just check if url of any type (but be careful of relative urls) and punt if so.
So maybe a simple file existance or is readable test here is the safest approach.