if (confirm('<?php echo __("Do you want to Upload the file?"); ?>')) {
                $.blockUI({message: $('#displayBox'), fadeIn: 1000,
                    css: {left: '33%', width: '36%', padding: '10px'},
                    onBlock: function() {
                        $(window).bind('beforeunload', function() {
                            return("<?php echo __("Please don't press Refresh, Back & Forward buttons of the browser until the process is completed."); ?>");
                        });
                    }
                });
            }
            else {
                $.unblockUI();
                return false;
            }

------------------------------------------------------------------------------------------------------------------------------------------
html:
<!-Below div is used for pop up-->
<div id="displayBox" style="display:none">
    <h1><?php echo __('Your file is being uploaded'); ?><br>&nbsp;<?php echo __("Please don't press Refresh, Back & Forward buttons of the browser until the process is completed."); ?><br></h1> <?php echo $this->Html->image('ajax-loader.gif'); ?>
</div>