There is a rar in the root-folder of the game with the name _super_secret. It is password-protected. Size: 1 GB. Very curious indeed...

It's not hard to find, a quick google search finds it, and The Dark Decent has a similar file, But A Machine For Pigs doesn't

It contains things that can be considered spoilers, and the passwords given in parts for completing the different endings

Source
Super Secret file (The Dark Descent) | Amnesia Wiki | Fandom
The Super_Secret file is a .rar file located in the redist folder of Amnesia: The Dark Descent and is locked with a password only obtainable by getting all three endings of the game. After every ending, you will get a part of the password, and you must combine the letters and numbers in order to get the password.
Password:
lke271tyr299odn314

Source
Super Secret file (Rebirth) | Amnesia Wiki | Fandom
Like Amnesia: The Dark Descent, the .rar contains concept arts, development images, and even few scrapped materials for Rebirth
Password
7UI84S3D3AN2XJ36E7G5J26KA3Y9