TYPO3 Core

There is problem with displaying thumbnails in the Backend in Internet Explorer.

It turned out to be the header X-Content-Type-Options: "nosniff" in combination with the code in typo3\sysext\backend\Classes\View\ThumbnailView.php

header('Content-type: image/' . $outext);

This sends Content-type: image/jpg which is a wrong, undefined content type, correct is Content-type: image/jpeg.

IE sees the invalid content type and is not allowed to sniff the content - resulting in handling it as application/octetstream and offering it as download.

This results in thumbnails not showing up in backend (black/red X instead).