Allow overriding the filename in content-disposition headers

This is useful, for example, if you have your payload output set to file and want to respond with the temporary file, but specifying a real filename. Something like the following:

```js
server.route({
  method: 'POST',
  path: '/',
  config: {
    payload: {
      output: 'file'
    },
    handler: function (request, reply) {
      reply.file(request.payload, { mode: 'attachment', filename: 'something.json' });
    }
  }
});
```