https://es.stackoverflow.com/questions/69424/uncaught-typeerror-o-contenttype-indexof-is-not-a-function

formulario.html

<script src="https://code.jquery.com/jquery-3.2.1.min.js"></script>
<script>
  $(document).ready(function() {
    $("body").on("submit", "form", function(event) {
        event.stopPropagation();
        event.preventDefault();

        var type = "post";
        var url = $(this).prop("action");
        var cache = false;
        var contentType = 'application/x-www-form-urlencoded';
        var processData = true;

        if (this['files[]'].files.length) {
            var data = new FormData(this);
            contentType = false;
            processData = false;
        } else {
            var data = $(this).serialize();
        }
        $.ajax({
            url: url,
            data: data,
            type: type,
            contentType: contentType,
            processData: processData
        }).done(function(data) {
            $(".section").html(data);
        });
    });
});
</script>
<form id="form" action="formulario.php" method="post">
    <input type="text" placeholder="Pon algo..." name="campo" /><br />
    <input type="file" name="files[]" multiple /><br />
    <input type="submit" value="Enviar formulario" />
</form>
<pre class="section"></pre>

formulario.php

<?php
var_dump($_POST);
var_dump($_FILES);
var_dump(file_get_contents('php://stdin'));