I have always kept /stored my playmos some since 1978 as i was crazy about preserving them and keeping them intqact from the age of 6 i suppose in cool, dry places free of moisture and high temps. I have had some playmos stored in a large shed when i was in the UK for years, rather low temperatures and i suppose lots of moisture on the outside, the shed was not insulated but was very new and no water went ewver though in it. All playmos were fine and as good as new. I dont knwo about extreme temperatures my guess is that they will affect the klickies !!!!
My biggest advice would be to keep and store the playmos in darkness, light affects the color quality and stickers etc etc and i think its the biggest ageing threat to playmobil. All my collections are in dark storage rooms and i think thats a key issue to consider apart from the moistures
I woudl imagine you attic is not that different from my garden shed in the UK . I had all my playmobil boxes and opened ones wrapped in plastic bags and they were all 100% as new when i opened them