If a file gets called it will be redirected to index.html
But not if it the requested File does contain a dot (and with this does have a file extension.
If you call:
links.domain.de/notexistingfile.jpg
It will trigger 404
If you call:
links.domain.de/server/[CODE-CODE-CODE]/list-short-urls/1
It will redirect the call to index.html