The original thread confused the issue of the server misconfiguration and the issue of an obsolete browser, and got banished to the How Do I...? forum.
But the server misconfiguration persists.
PNG images like https://www.geocaching.com/images/stars/stars3.gif use a .gif filename extension, and are being sent with the HTTP header
It might be nice to rename them to use a .png filename extension, but it is critical that they be sent