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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 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)