Hi Marc,
For displaying the file icons based on the file extension
For changing the theme from light to dark mode, add CSS classes to change the colour of the SVG rendered.
Then add a conditional class to the images based on an attribute that checks which mode the page is currently in (dark of light).
See Mendix Forum - Question Details
https://marketplace.mendix.com/link/component/108388/Webflight/File-Type-Checker
For adding the images in bulk, you can build yourself a page that stores the images to your DB. Alternatively, you can import your images from online. Try google icons.
https://www.w3schools.com/icons/google_icons_intro.asp
Material Icons Guide | Google Fonts | Google Developers
Hope that helps!