Brave hundreds of hand-crafted challenges as you help Madeline survive her journey to the top of Celeste Mountain! · By Maddy Makes Games, Noel, Heidy Motta, saint11
They're custom run-length encoded bitmap data, but no sprite editor will open them by default. However, some fans have made scripts to turn them into PNG files (in the Celeste speedrunning discord under the #game_modding pinned posts).
but how do you open the files