framework/web/Response.php
public function setDownloadHeaders($attachmentName, $mimeType = null, $inline = false, $contentLength = null)
    {
        $headers = $this->getHeaders();

        $disposition = $inline ? 'inline' : 'attachment';
        $headers->setDefault('Pragma', 'public')
            ->setDefault('Accept-Ranges', 'bytes')
            ->setDefault('Expires', '0')
            ->setDefault('Cache-Control', 'must-revalidate, post-check=0, pre-check=0')
            ->setDefault('Content-Disposition', $this->getDispositionHeaderValue($disposition, $attachmentName));

        if ($mimeType !== null) {
            $headers->setDefault('Content-Type', $mimeType);
        }

        if ($contentLength !== null) {
            $headers->setDefault('Content-Length', $contentLength);
        }

        return $this;
    }
public function setDownloadHeaders($attachmentName, $mimeType = null, $inline = false, $contentLength = null)
    {
        $headers = $this->getHeaders();

        $disposition = $inline ? 'inline' : 'attachment';
        $headers->setDefault('Pragma', 'public')
            ->setDefault('Accept-Ranges', 'bytes')
            ->setDefault('Expires', '0')
            ->setDefault('Cache-Control', 'must-revalidate, post-check=0, pre-check=0')
            ->setDefault('Content-Disposition', $this->getDispositionHeaderValue($disposition, $attachmentName));

        if ($mimeType !== null) {
            $headers->setDefault('Content-Type', $mimeType);
        }

        if ($contentLength !== null) {
            $headers->setDefault('Content-Length', $contentLength);
        }

        return $this;
    }
Sets a default set of HTTP headers for file downloading purpose.
@param string $attachmentName the attachment file name
@param string $mimeType the MIME type for the response. If null, `Content-Type` header will NOT be set.
@param bool $inline whether the browser should open the file within the browser window. Defaults to false,
meaning a download dialog will pop up.
@param int $contentLength the byte length of the file being downloaded. If null, `Content-Length` header will NOT be set.
@return $this the response object itself