I think ?chunks? in this context is referring to chunked file encoding. Chunked encoding gets used when the entire payload size isn?t known in advance, like is a multi-part database query, so the size is provided progressively in each piece sent. When sending a file where the length is known, chunked encoding isn?t necessary.
This is not the same thing as streaming the file 1k or 50k bytes at a time, or TCP?s packetizing.