I tried this solution and added the following to my apache2.conf file:
<Directory /var/www/mediawiki/images>
AllowOverride None
AddType text/plain .html .htm .shtml .phtml
<IfModule LiteSpeed>
php_admin_flag engine off
</IfModule>
</Directory>
I added <IfModule LiteSpeed>, </IfModule> tags according to this because otherwise my apache2 server shows the syntax error on the line where php_admin_flag is.
I also tried to add the following to the/var/www/mediawiki/images/.htaccess file:
AddType text/plain .html .htm .shtml .phtml .php .php3 .php4 .php5 .php7
RemoveHandler .php
<FilesMatch "\.ph(p[3457]?s?|tml)$">
SetHandler None
</FilesMatch>
but it did not help.
I changed the server API from Apache 2.0 Handler to FPM/FastCGI as this is one of the MediaWiki's requirements.
What else can I do to solve that security problem? The help is appreciated.