内部服务器错误Codeigniter Ajax上传文件

Question

我可以通过ajax发送数据，但服务器返回错误：500 POST sing_in。

我认为问题是配置：
$config["upload_path"] = './uploaded_files';但不是。

你认为这里的问题是什么，因为我什么都不知道！

控制器：

public function sing_in(){

            echo $_FILES['foto_user']['name'];

            $config["upload_path"]   = './uploaded_files';
            $config["allowed_types"] = 'gif|jpg|png';


            $this->load->library("upload",$config);

            if($this->upload->do_upload('foto_user')){

                echo  "upload";

            }else{
                echo $this->upload->display_errors();
            }


        }

使用Javascript：

$(document).ready(function(){


    var url ='<?php echo base_url()."index.php/inicio/sing_in";?>';


    $('#sing_form').on("submit",function(e){

        e.preventDefault();
            //console.log("submited");


                $.ajax({
                    url: url,
                    method: "POST",
                    data: new FormData(this),
                    contentType: false,
                    cache:false,
                    processData: false,
                    success: function(data){
                      console.log(data);
                    },error:function(data){
                        console.log(data)
                    }
                });

    });     


});

我的观点：

/**
*TABLE SING IN
**/
$this->table->set_heading("","Sing in");
$this->table->add_row(form_label("Username: "),form_input('name_user'));
$this->table->add_row(form_label("Password: "),form_password("password_user"));
$this->table->add_row(form_label("Repeat Password: "),form_password("rpassword_user"));
$this->table->add_row(form_label("Department"),generate_dropdown("department",$departments_results));
$this->table->add_row(form_label('Foto'),form_upload("foto_user"));
$this->table->add_row("",form_submit("sing_in","Sing in"));

echo '<div id="container_table_sign">';
echo form_open_multipart("",'id="sing_form"','name="sing_form"','method="POST"');
echo $this->table->generate();
echo '</div>';