Checks if img_id has real file on filesystem.

def is_img_id_exists(img_id):
    """
    Checks if img_id has real file on filesystem.
    """
    main_rel_path = get_relative_path_from_img_id(img_id)
    main_path = media_path(main_rel_path)
    return os.path.isfile(main_path)