A delegate used to specify the extensions for each chunk written to the underlying stream.
The delegate has to be of type
string delegate(in const(ubyte) data) and gets handed the
data of each chunk before it is written to the underlying stream. If it's return value is non-empty,
it will be added to the chunk's header line.
The returned chunk extension string should be of the format
and **not contain any carriage return or newline characters**.
Also note that the delegate should accept the passed data through a scoped argument. Thus, **no references to the provided data should be stored in the delegate**. If the data has to be stored for later use, it needs to be copied first.