Natsuki's File
You have to rename this file into a .jpeg file (natsuki.jpg).Opening it will result in this odd discolored image:
But to anyone that has experience with photoshop,it's easy to invert the colours of this picture.Then wrapping it around a 3D cone and looking on it from above will result in this view of picture showing a blond/white-haired women with whited-out eyes:
This women has never been seen before in DDLC,so it might be a character from Project:Libitina.Maybe it's Elyssa,the one we heard from in the last secret poem.But there's no proof yet.
Here's a great post with ALL the spoilers / Easter eggs:
https://indiefaq.com/guides/134-doki-doki-literature-club.html