PHP doesn't get $_FILES when sending form data using Ajax

I'm trying to create an upload element on my website. What I'm trying to do is an upload button that instantly when the user picks a file from the computer it uploads it to the server.

This is my form:

<form id="createAlert" name="createAlert" enctype="multipart/form-data" method="POST" action="createAlert.php">
    <input name="title" type="text" class="input-medium" maxlength="36"></input>

    <textarea name="content" rows="7" cols="90" class="input-short" style="font-family: arial; width: 80%; resize: none; height: 250px"></textarea>

    <input name="push" type="checkbox"></input>

    <input enctype="multipart/form-data" name="img" id="img" size="35" type="file"/>

    <input class="submit-gray" type="submit" value="POST ALERT"/>
</form>

Here is my Javascript code that sends the file as a FormData object to an upload page (upload.php):

    $('#img').live('change', function() {
        var formData = new FormData();
        formData.append('file', $('#img')[0].files[0]);
        $.ajax({
            url : 'upload.php',
            type : 'POST',
            data : formData,
            enctype: 'multipart/form-data',
            success : function(data) {
                console.log(data);
                alert(data);
            },
            cache: false,
            contentType: false,
            processData: false
        });
    });

So far, so good - everything works fine. The problem is with the upload.php file that receives the FormData. Here is it's code (It's only a testing version it doesn't upload the file yet):

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

The problem is that the output of var_dump($_FILES); and var_dump($_POST) is two empty arrays, while in the file_get_contents('php://input') I get the file data.

Here is the output (I cut out the part with the content of the uploaded file...):

------WebKitFormBoundaryw4nmFcISqYuAWQOS
Content-Disposition: form-data; name="file"; filename="Sem Título-1.png"
Content-Type: image/png

//Here is the file I uploaded...

------WebKitFormBoundaryw4nmFcISqYuAWQOS--
array(0) {
}
array(0) {
}

I read dozens of answers to questions here and many solutions to the problem I have but none of them solved the problem.

What am I doing wrong? Why am I receiving the file just in the php://input but not using $_FILES?

Thank you!