nsIFileURL provides access to the underlying nsIFile object corresponding to
an URL. The URL scheme need not be file:, since other local protocols may
map URLs to files (e.g., resource:).
Get/Set nsIFile corresponding to this URL.
Getter returns a reference to an immutable object. Callers must clone
before attempting to modify the returned nsIFile object. NOTE: this
constraint might not be enforced at runtime, so beware!!
Setter clones the nsIFile object (allowing the caller to safely modify
the nsIFile object after setting it on this interface).