JavaScript multipart-form data is not retrievable to PHP codeigniter backend-ThrowExceptions

I’m trying to submit a form with files using javascript to a PHP Codeigniter backend. The javascript code looks like this,

            var card = {
                        user: {
                            my: {
                                firstname: this.user.my.firstname,
                                lastname: this.user.my.lastname 
                            }
                          }
                        }
            var form = new FormData()
            form.append('card', card)
            form.append('file', this.user.cover_file)

            axios({
                method: 'post',
                headers: {'Content-Type': 'multipart/form-data'},
                data: form,
                url: 'url'
            })

And I am unable to retrieve the data from the PHP backend, the backend code looks like this,

public function newpost()
{
    $this->load->database();
    $user = $this->input->post('user');
    echo $user;
}

This does not show any output, what am I doing wrong here and why is the data not retrivable from the backend, thanks