Implement io.Writer for File

It would be nice if `type File` would implement `io.Writer`. That way I could update some static files on the fly when served via `http.FileServer(box.HTTPBox())`.

Start by locating the type File and its existing read-related methods in the repository, then check Go's io.Writer contract and how File is used with http.FileServer(box.HTTPBox()). Done means File satisfies io.Writer and the behavior is covered by appropriate tests.

Written by the indexing model from the issue text.