In your supplementary question you asked if www.example.com/file[/].html is a valid URL.
That URL isn't valid because a URL is a type of URI and a valid URI must have a scheme like http: (see RFC 3986).
If you meant to ask if http://www.example.com/file[/].html is a valid URL then the answer is still no because the square bracket characters aren't valid there.
The square bracket characters are reserved for URLs in this format: http://[2001:db8:85a3::8a2e:370:7334]/foo/bar (i.e. an IPv6 literal instead of a host name)
It's worth reading RFC 3986 carefully if you want to understand the issue fully.
