Appending a value to metabox before and after saving

I was able to get it working by adding this…

jQuery(document).ready(function($){

    var custom_uploader;
    var currentfiles = document.getElementById('my_file_URL').value;
    var currentID = document.getElementById('my_file_ID').value;

…..

                var urls = selection.map( function(attachment){
                    return attachment.url;
                });
                var ids = selection.map( function(attachment){
                    return attachment.id;
                });


                $( '#my_file_URL' ).val(urls.join(','));
                $( '#my_file_ID' ).val(ids.join(','));

                if(currentfiles != ''){
                    $( '#my_file_URL' ).val(currentfiles + "," + urls);
                }

                if(currentID != ''){
                    $( '#my_file_ID' ).val(currentID + "," + ids);
                }