Empty $_FILE while sending image with jQuery ajax [duplicate]

I am trying to upload an image file to the server, but the $_FILE is always empty and I don't know why.

Here is the html:

    <form action='' id='ajax_form' method='post' enctype="multipart/form-data">
      <input type='file' name='imagem'>
      <input type='submit' id='btUploadImagem' value='Upload'>
    </form>

And here the jquery script:

    function salvaImagem(){

    var form = $(this).closest('form'); 
    var formData = new FormData(form);
    var dados = formData;
    jQuery.ajax({
        type: "POST",
        url: "salvaImagem",
        data: dados,
        contentType: false,
        processData: false,
        dataType: "json"}).done(function(response)
        {
            if(response.sucesso){
                //show success message
            }
            else{
                //show error message
            }
        });
    return false;
}

$('#btUploadImagem').on('click', salvaImagem); 

And the controller php:

    public function salvaImagem()
    {   
       $arquivo = isset($_FILES["imagem"]) ? $_FILES["imagem"] : FALSE;
       //some other checks
    }

What am I doing wrong so the $_FILE is always empty?