Fileupload - not retrieving server error response - kept return 'success'

Question

My autoUpload set as false & I want to handle server error in frontend ( when I upload .gif or .mp3 ) using fileupload's error handler.

But the server response/textStatus always return success although my Php codes set accept_file_types as jpg|png.

Javascript codes :

$('#fileupload').fileupload({
    url              : 'sample/api/upload/',
    autoUpload       : false,
    maxChunkSize     : 50000, //5mb
    add: function (e, data) {
        var that = this;
        $.blueimp.fileupload.prototype.options.add.call(that, e, data);
        $("#submit_btn").off('click').on('click', function () {
            data.submit()
                .success(function (result, textStatus, jqXHR) { alert('success') })
                .error(function (jqXHR, textStatus, errorThrown) { alert('error') })
                .complete(function (result, textStatus, jqXHR) { alert('complete') });
        });
    }
});

  Php codes :

require('/UploadHandler.php');
$upload_handler = new UploadHandler(array(
    'upload_dir'        => 'sample/assets/uploads/tmp/',  
    'upload_url'        => 'http://localhost/sample/assets/uploads/
    'accept_file_types' => '/\.(jpe?g|png)$/i',
));

 

Ref :

• ref1 : autoUpload trigger upload by btn
• ref2 : configure fileupload's upload directory in php

Anyway to retrieve server fail response with autoUpload:false? Tq..