function sortByExt(files) {
return files.map(fileName => {
const p = fileName.indexOf('.');
const name = +fileName.slice(0, p);
const ext = fileName.slice(p + 1);
return [fileName, name, ext];
}).sort(([, nameA, extA], [, nameB, extB]) => (extA < extB
? 1
: (extA > extB ? -1 : nameA - nameB)
)).map(([fileName]) => fileName);
}