upload.js

document.getElementsByClassName("btn")[0].addEventListener("click", function () {

    var arquivos = document.getElementsByClassName("arquivos")[0].files;
    var dados = new FormData();
    var url = new XMLHttpRequest();

    for (var i = 0; i < arquivos.length; i++) {
        dados.append("arquivo[]", arquivos[i]);
    }

    url.open("POST", "/upload.php");
    url.send(dados);

    url.onreadystatechange = function () {
        if (url.readyState == 4) {

            if (url.status == 200) {
                document.getElementsByClassName("alert-success")[0].innerHTML = '<span class="glyphicon glyphicon-exclamation-sign" aria-hidden="true"></span><span class="sr-only">Error:</span> Enviado com sucesso';
                document.getElementsByClassName("alert-success")[0].style.display = "block";
            }

            if (url.status == 404) {
                document.getElementsByClassName("alert-danger")[0].innerHTML = '<span class="glyphicon glyphicon-exclamation-sign" aria-hidden="true"></span><span class="sr-only">Error:</span>  Ooops houve um erro: 404';
                document.getElementsByClassName("alert-danger")[0].style.display = "block";
            }

        }
        document.getElementsByClassName("miniaturas")[0].innerHTML = "";
    }

});