Big File Support Spec

Please provide the specification for the big file support feature. If you don't know where to start, here are some example topics to write about.

**How big files are hashed. How merkle trees are made.**

Piece size, hashing algorithm, number of leaf nodes, etc.

**How a big file is represented in content.json**

Piece field format, hashing algorithm, keywords, etc.

**Does big file support introduce changes to the network protocol?**

Are big files transmitted over the current network protocol? Is the network protocol changed?

**What are the preferred ways to store an incomplete big file?**

How did you do it.

**The status of the current specification.**

Draft, pending review, recently revised, final version, etc.