I imagine this would depend on the .svg, as they do not necessarily have an explicit width set initially. By nature they are vector-files, so size is irrelevant, and I think the image-processing back-end (Gregwar) checks file metadata for this - which for a .svg would as you note, return null, because it is only set through HTML-attributes and are generally considered superfluous (control .svg-size through CSS, not HTML).