When using highlevel request() api, users can control the block size by
wrapping the file object with an iterator:

    class FileIter:

        def __init__(self, file, blocksize):
            self.file = file
            self.blocksize = blocksize

        def __iter__(self):
            while True:
                datablock = self.file.read(self.blocksize)
                if not datablock:
                    break
                yield datablock

Adding configurable block size will avoid this workaround.