Lets the view load the correctly scaled icon on demand.
KIO::pixmapForUrl and KIconLoader::loadMimeTypeIcon says something about "loading additional icon themes" but they're hardly used in KIO anymore so it's likely the file list itself would have a hard time finding the icons anyway, so we can just use QIcon::fromTheme here
loadMimeTypeIcon also has the fallback to application-octet-stream if the mimetype icon isn't found, you could pass that as 2nd argument to fromTheme().
As to the extra desktop themes, I admit that I never really understood what that was about. KIconLoaderPrivate::addExtraDesktopThemes() seems to add *all* icon themes available on the system and named default.* (other than default.kde)... Which use case does this cover? And anyway if any kiconloader does this then fromTheme() will end up there too (just like the code for mimetype-generic-icons, which is hopefully also triggered when going via fromTheme....)