I have form upload:
 <form method="post" id="edit_form" onsubmit="return edit(this.id);" enctype="multipart/form-data">
    <input type="file" name="thumb" />
    <input type="submit" value="Upload" />
</form>
and javascript function:
function edit(id){
    httplocal = location.href;
    var jqxhr = $.post(httplocal,$("#"+id).serialize(), function(data) {
        if(data.status == 1){
            $("#success").show();
        }else {
            $("#error").show();
        }
    }, "json")
    return false;
}
in php i check :
if ($_FILES['thumb']['error'] != UPLOAD_ERR_NO_FILE) {
    code upload here...
}
but form is empty $_FILES['thumb'] is null. Sorry my english.
 
     
    