yii\mongodb\file\Upload::addStream PHP Method

    public function addStream($stream)
    {
        while (!feof($stream)) {
            $freeBufferLength = $this->chunkSize - StringHelper::byteLength($this->buffer);
            $streamChunk = fread($stream, $freeBufferLength);
            if ($streamChunk === false) {
                break;
            }
            $this->buffer .= $streamChunk;
            $this->flushBuffer();
        }
        return $this;
    }