This is a bit odd, don't you want to set it to application/octetstream if it is application/octet-stream? Right now you're not doing anything if it is application/octet-stream.

If so, would be less confusing with:
$this->mimetype = $this->mimetype_guesser->guess($filename);
if ($mimetype === 'application/octetstream') {
$mimetype = 'application/octetstream';
}