system/HTTP/DownloadResponse.php
public function buildHeaders()
	{
		if (! $this->hasHeader('Content-Type'))
		{
			$this->setContentTypeByMimeType();
		}

		$this->setHeader('Content-Disposition', $this->getContentDisposition());
		$this->setHeader('Expires-Disposition', '0');
		$this->setHeader('Content-Transfer-Encoding', 'binary');
		$this->setHeader('Content-Length', (string)$this->getContentLength());
		$this->noCache();
	}
public function buildHeaders()
	{
		if (! $this->hasHeader('Content-Type'))
		{
			$this->setContentTypeByMimeType();
		}

		$this->setHeader('Content-Disposition', $this->getContentDisposition());
		$this->setHeader('Expires-Disposition', '0');
		$this->setHeader('Content-Transfer-Encoding', 'binary');
		$this->setHeader('Content-Length', (string)$this->getContentLength());
		$this->noCache();
	}
set header for file download.