When trying to include SVGs using the extension.json RessourceLoader fails to create valid CSS when the SVG is too big.
ResourceLoaderImageModule::getCssDeclarations tries to set a data URI and embed the SVGs inline. But when the SVG is too big CSSMin::encodeStringAsDataURI returns false leading to invalid data there and and an empty background-image: url();.
The later ( although faulty ) overrides the rasterized fallback and no image shows at all.
Possible solutions ( to be discussed )
- do not embed SVG inline when to big but reference via path
- use rasterized fallback
 see CSSMin::DATA_URI_SIZE_LIMIT = 32768;