end_del.html

<form id="form" action="" method="post">
    <input id="formdata" type="hidden" name="files">
</form>

<script>
    let button=document.getElementById("button");
    button.style.display="none";
    let form=document.getElementById('form');
    let formdata=document.getElementById('formdata');
    let files=[];
    let n=0;
    function select_(div){
        var im = div.children[0];
        if (im.className=="selected"){
            im.className="unselected";
            files.splice(files.indexOf(im.src), 1);
            n-=1;
            if(n==0){
                button.style.display="none";
            }
        } else{
            im.className="selected";
            n+=1
            button.style.display="block";
            files.push(im.src.replace("_thumbs","").replace("_square",""));
        }
    }

    function sendit(){
        formdata.value=files;
        form.submit();
//        for(var i=0;i<files.length;i++){
//            formdata.value=files[i].replace("_thumbs","").replace("_square","");
//            form.submit();
//        }
    }

</script>
</body></html>