Made check for MIME type "image/svg+xml" case insensitive

It was possible to bypass this check by sending a manipulated HTTP request with a MIME type like "image/svg+XML".
This check was originally implemented to address CVE-2022-30110.

Reported by:
- Yann CAM (ycam) (https://yann.cam/)
- Georges TAUPIN (jo) (https://www.georgestaupin.com/)