@Ellie ******
The STV and the extensions are such a mess that my advice is simply **ask the immigration office what their requirements are for that extension of stay**. Some require the same paperwork as a regular extension of stay based on tourism, and others require the same documents as the original visa.
Nobody knows what will happen with the STV in the future either.
(IF anyone responds to this comment with a "well, what about my STV?", I'm deleting the comment; make your own post, or read this comment around the stars).