framework/web/Response.php
public function sendContentAsFile($content, $attachmentName, $options = [])
    {
        $headers = $this->getHeaders();

        $contentLength = StringHelper::byteLength($content);
        $range = $this->getHttpRange($contentLength);

        if ($range === false) {
            $headers->set('Content-Range', "bytes */$contentLength");
            throw new RangeNotSatisfiableHttpException();
        }

        list($begin, $end) = $range;
        if ($begin != 0 || $end != $contentLength - 1) {
            $this->setStatusCode(206);
            $headers->set('Content-Range', "bytes $begin-$end/$contentLength");
            $this->content = StringHelper::byteSubstr($content, $begin, $end - $begin + 1);
        } else {
            $this->setStatusCode(200);
            $this->content = $content;
        }

        $mimeType = isset($options['mimeType']) ? $options['mimeType'] : 'application/octet-stream';
        $this->setDownloadHeaders($attachmentName, $mimeType, !empty($options['inline']), $end - $begin + 1);

        $this->format = self::FORMAT_RAW;

        return $this;
    }
public function sendContentAsFile($content, $attachmentName, $options = [])
    {
        $headers = $this->getHeaders();

        $contentLength = StringHelper::byteLength($content);
        $range = $this->getHttpRange($contentLength);

        if ($range === false) {
            $headers->set('Content-Range', "bytes */$contentLength");
            throw new RangeNotSatisfiableHttpException();
        }

        list($begin, $end) = $range;
        if ($begin != 0 || $end != $contentLength - 1) {
            $this->setStatusCode(206);
            $headers->set('Content-Range', "bytes $begin-$end/$contentLength");
            $this->content = StringHelper::byteSubstr($content, $begin, $end - $begin + 1);
        } else {
            $this->setStatusCode(200);
            $this->content = $content;
        }

        $mimeType = isset($options['mimeType']) ? $options['mimeType'] : 'application/octet-stream';
        $this->setDownloadHeaders($attachmentName, $mimeType, !empty($options['inline']), $end - $begin + 1);

        $this->format = self::FORMAT_RAW;

        return $this;
    }
Sends the specified content as a file to the browser.

Note that this method only prepares the response for file sending. The file is not sent
until [[send()]] is called explicitly or implicitly. The latter is done after you return from a controller action.

@param string $content the content to be sent. The existing [[content]] will be discarded.
@param string $attachmentName the file name shown to the user.
@param array $options additional options for sending the file. The following options are supported:

- `mimeType`: the MIME type of the content. Defaults to 'application/octet-stream'.
- `inline`: boolean, whether the browser should open the file within the browser window. Defaults to false,
meaning a download dialog will pop up.

@return $this the response object itself
@throws RangeNotSatisfiableHttpException if the requested range is not satisfiable
@see sendFile() for an example implementation.