Send a File with Ajax into prototype

I'm trying to send a File to the Server without success. I have this :

HTML(it's not within a form ):

<td class="td_logo">

<input type="file" accept="image/*" multiple="false" name="realField" id="realField">

</td>

and i have a button who trigger the JS code.

JS

function sendRequest(url, firmObj){
    new Ajax.Request(url,   
    {   
        method: 'post',   
        postBody: 'firmObj='+ Object.toJSON(firmObj),  
        onSuccess: function(){}
    });  
}

and the url call the PHP.

PHP

if( isset($_POST["firmObj"]))
{
    $firmObj = json_decode($_POST["firmObj"]);
    var_dump($firmObj); 
}else{ echo "FAIL"; }

if($_FILES['realField']['name'])
{
    print_r($_FILES['realField']['name']);
}else{ echo "FAIL";}

var_dump($firmObj) jus开发者_运维百科t show me the file name and the other inputs in my table... but $_FILES['realField']['name'] is always false and shout "FAIL" out!!

How can i get the $_FILES filled with input data into prototype?

Thank you!!

Javascript cannot read local files (it would be a security nightmare) so AJAX doesn't really support uploads. Scripts like this one work by creating a form in a hidden iframe and submitting that.