var files = [], slide='', str='';
for (let n = 0; n < 28; n++) {
    files[n]=n+1;
}
var len=files.length-1,endlen = files.length>32 ? files.length-32 : 0;
var i=0;
while (len>=endlen) {
    str+=`<li>${files[len]}</li>`;
    i++;
    if(i%8==0 || len==endlen){
        slide+=`<ul>${str}</ul>`;
        str='';
    }
    len--;
}
$('body').append(slide)

每8个为一组，最多共4组，倒序排列。

正序循环分组

var files = [];
var slide = '', str = '';
for (let n = 0; n < 12; n++) {
    files[n]=n+1;
}
for (var i = 0; i < files.length; i++) {
    str += `<li data-index="${i}">${files[i]}</li>`;
    if ((i+1) % 5 == 0 || i == files.length-1) {
        slide += `<ul>${str}</ul>`;
        str = '';
    }
}
$('body').append(slide)