In client side:

HTML:

<form id="FILEFORM" enctype="multipart/form-data">
    <input id="FILE" name="ADDFILE[]" type="file" onchange={this.handleFiles} multiple/>
</form>

The magic key here is to set the name "ADDFILE[]". Without "[]", the PHP side will only receive single file.
Also we need to add multiple to <input>.

Javascript:

var formData = new FormData($('#FILEFORM')[0]);
/* equivalent to above code. But in this way we can rename file names for ever files we selected.
var formData = new FormData();
$.each( $('#FILEFORM').find('input')[0].files, function( i, file ){
    formData.append('ADDFILE', file );
});
*/
$.ajax({
    url: 'mod.php',  //Server script to process data
    type: 'POST',
    data: formData,
    cache: false,
    contentType: false,
    processData: false //dont process since it's formdata.
})

$('#FILEFORM')[0] means get the original javascript DOM with Jquery because we need it for formData.

PHP:

$total = count( $_FILES['ADDFILE']['name'] );
for ( $i = 0; $i < $total; $i ++ )
{
    ... // process data..
}

The key here is to get the file with name ADDFILE which we defined in HTML <input> attr name.