Jonexys: What files i have to put there?
Ignore it completely if you're unsure. The attachment field is there always, also no matter what category or "Problem type" you select, and is used for screenshot of error messages and what ChrisGamer300 wrote.
Just give them the order ID, ask for refund, and submit. They need nothing more. If GOG support want more from you they usually ask for it ;)
EDIT: I recently requested a refund and just gave them the order ID and suggested wallet funds, and I got asked to confirm that I'm ok with wallet funds, and I got the money shortly after. There's usually no hassle with GOG support if you at least try to meet them half way ;)