As you can read in the post installation message that is new as of Joomla 3.9.3, "Since Joomla 3.9.3, Joomla is shipped with additional security hardenings in the default htaccess.txt and web.config.txt files. These hardenings disable the so called MIME-type sniffing feature in webbrowsers. The sniffing leads to specific attack vectors, where scripts in normally harmless file formats (i.e. images) will be executed, leading to Cross-Site-Scripting vulnerabilities.
The security teams recommends to manually apply the necessary changes to existing .htaccess or web.config files, as those files can not be updated automatically." How to manually apply these necessary changes to prevent browsers from sniffing your Joomla site is what today's Maintenance Monday is all about.
