User specified block-splitting functions

Hi,

In the white paper, secion 3.6.7 Splitting files into Lists and Blob, it mentions about IPFS: "Allow users to specify block-splitting functions highly tuned for specific files.

Is this implemented in the codebase? The closest I can see in terms of functionality of taking input and creating the blocks/hashes is js-ipfs-unixfs-engine (https://github.com/ipfs/js-ipfs-unixfs-engine)

Thanks!