KML/KMZ loading breaks for `svg+xml` images

Sandcastle example: working on it

Browser: All?

Operating System: All?

I managed to make an entity collection that, when exported with `exportKml`, doesn't load back into Cesium correctly. I'm working on a boiled down repro / Sandcastle of this, but the short version is that each entity has a Billboard whose image is data URL containing `svg+xml`. The exported KMZ includes one SVG file for each entity, with a `.svg+xml` extension. I believe the resulting KMZ would load fine in Google Earth, but I don't have it handy for testing.

The issue is, it fails to render correctly when loaded in a KmlDataSource, because the billboards are created with an image whose value is `data:application/octet-stream;base64...`, where the base64 encoded data is the entire SVG file as a literal. As an experiment, I changed the `billboard.image._value.url` property in the console so that it uses `data:image/svg+xml;base64,...` instead, and immediately the correct image showed up on the map.

TL;DR: it sounds like KmlDataSource can't render placemarks with a `.svg+xml` filetype, even though `exportKml` will produce them. (The fact that it's loading from a KMZ instead of KML might be important, because you lose the benefit of server-specified MIME types for the image.)

Start with KmlDataSource's KMZ image handling and compare it with the exportKml output described in the issue. Reproduce the case in a Sandcastle example using an entity Billboard with a data URL containing svg+xml; the work is done when the exported KMZ reloads in KmlDataSource and renders its SVG billboards correctly.

Written by the indexing model from the issue text.