Improve ":id in file_path" and its documentation

[Allyedge]

So, I tried to add an "update user avatar" feature in my app with this library today, and it didn't really work because the user creation and the avatar insertion parts are completely different pages.

It would be really great if the documentation gets improved there because I personally couldn't find a way to have a special storage_dir for user's avatars since the scope is usually nil because the creation of the user is a different part of the app.

Thanks! 🙂