@andi: Even if you told me not to do so, I completely uninstalled apache2 and the related PHP7.4 by
sudo apt-get purge apache2 php php-gd php-xml php-json
Of course I saved all data in /var/www
and the config-files in /etc/apache2
Then I reinstalled everything again:
sudo apt-get install apache2 php php-gd php-xml php-json -y
First I enabled my little html test page, containing diverse images (SVG, PNG, GIF, JPG, ICO), called it in the browser and... ALL images appeared!
wget/<path-to-page/<pic>.svg
now reports as type image/svg+xml
as it should and the same is true for the other image-types as all had been shown as text/html
.