File objects should have a "readOnly" property, indicating whether write permission is granted by the user. Files returned from <input> elements should, by default, set it. Constructing a FileWriter using a File with its readOnly property set should throw an exception.
Later, it would be good to also have an attribute on <input> to allow opening files for write. For example, <input type=file file=write> to ask for write permission (setting readOnly if only read permission was granted, eg. with an "open read-only" checkbox), and <input type=file file=create> to also ask the browser to show a "Save As" dialog instead of an "Open" dialog. -- Glenn Maynard