Some plugins come with their own demo surveys or questions/groups. It looks like LS 6.x blocks these files during plugin import without notice. But I am pretty sure this worked fine at some older versions as well as LS 3.x and 5.x because we have several plugins with a demo survey shipped.

Config shows that lss, lsa, lsg, lsq is missing at:
'allowedpluginuploads' => 'gif,ico,jpg,png,css,js,map,json,eot,otf,ttf,woff,txt,md,xml,woff2,twig,php,html,po,mo,xsd