Yes and I get the same error with the default ISIS admin template, so I assume then this is a Joomla issue? I am able to use svg files on my pages and on my site template. Is there some parameter I need to add this support for svg on the admin side?
I upload the svg to my images folder and then I go into the template, advanced tab, Login Page tab (really any of them) and click select for the image. In the images folder it will not show any svg file. I tried to click the upload and select the svg image and when I do it tells me this image format is not supported. I tried to drag the image into the upload area and also tried browse and add, neither works and throws the error alert popup up.