Stu is correct, in 99,9999% we'll reject it, BUT it's also OK to use an existing storage handler like core_Theme in few cases like uploading a watermark image, logo or any other use cases where it's not worth to have an own file storage extension and database table and all the other logic, as long as you take care of ALL the logic like removing the file when the plugin is removed, updating the path when the files were moved or the storage handler was changed.
We have also logic in place ( see Application::uploadSettings() ) to register the settings so that the framework takes care of everything.