Returns true if the function as-file would return the name of an existing file, and false if as-file would create a temporary file.
For example, it returns true if the stream was created from a file using the stream-open function or returned from a mapping. If the stream is not served from a file or it is a file but a temporary one, this function returns false.
is-file(s:stream) -> Boolean
Specifies the input stream.