CodeIgniter的文档明确指出，您需要定义文件上传所允许的文件类型。如果您不这样做，则没有默认的允许类型列表。
文档中的相关代码可以在下面找到。特别是$config数组的'allowed_types'元素。

function do_upload()
{
    $config['upload_path'] = './uploads/';
    $config['allowed_types'] = 'gif|jpg|png';
    $config['max_size'] = '100';
    $config['max_width']  = '1024';
    $config['max_height']  = '768';

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

    if ( ! $this->upload->do_upload())
    {
        $error = array('error' => $this->upload->display_errors());

        $this->load->view('upload_form', $error);
    }
    else
    {
        $data = array('upload_data' => $this->upload->data());

        $this->load->view('upload_success', $data);
    }
}