So far so good - I can't see the svg in de media manager that opens when I click insert image, but it's easy enough to enter it manually.
However: I also want to insert it in the "logo" position of the template. And there, there is no possibility for manually editing the filename (as far as I can see).
In J3, this was easy, because the media manager showed the path+filename of the chosen image (and that was editable).
Oops, never mind. I needed to add svg as a valid filetype and image/svg+xml as a valid mime-type in de Media Options. Duh!