下载的文件有错误的mime-type

I have a PHP script that creates a Zip Archive and then passes it to the browser for download.

This can be easily done via HTTP Response Headers

header('Pragma: no-cache');
header('Expires: 0');
header('Cache-Control: must-revalidate, post-check=0, pre-check=0, public')
header('Content-Type: application/zip')
header('Content-Length: ' . filesize($zipPath))
header('Content-Disposition: attachment; filename=filename.zip')
header('Content-Transfer-Encoding: binary')

echo file_get_contents($zipPath);

The file is correctly downloaded. I can also open it. However, when I check the downloaded file mime-type, it tells me : application/octet-stream instead of application/zip.

For file mime-type detection, I use the following snippet :

$pathToDownloadedFile = 'somewhere-on-disk';
$file = finfo_open(FILEINFO_MIME_TYPE);
var_dump(
    file_exists($pathToDownloadedFile), 
    finfo_file($file, $pathToDownloadedFile)
);

which dumps out :

true
application/octet-stream

I googled the problem but with no luck.
Any piece of help would be very appreciated.