The sixth envelope contains the elements required for The Mine, Episodes 16, 17 & 18. Opening up the envelope will give you access to everything you need for the next 3 games. But I will reveal the information needed for each of those episodes one at a time, under the individual spoiler tags below.
Chapter 6 – The Mine
End of Chapter 6
That is the end of Chapter 6. The game carries on in Chapter 7: The Railroad.