jQuery(document).ready(function() {

    var formfield;

    /* user clicks button on custom field, runs below code that opens new window */
    jQuery('#upload_file_button').click(function() {
        formfield = jQuery('#upload_file').attr('name');
        tb_show('','media-upload.php?TB_iframe=true');

        return false;

    });

    // user inserts file into post. only run custom if user started process using the above process
    // window.send_to_editor(html) is how wp would normally handle the received data

    window.original_send_to_editor = window.send_to_editor;
    window.send_to_editor = function(html){

        if (formfield) {
            fileurl = jQuery(html).attr('href');

            if(fileurl!=null) {
                fileurl = jQuery(html).attr('src');
            }
            jQuery('#' + formfield).val(fileurl);

            tb_remove();
        } else {
            window.original_send_to_editor(html);
        }
    };

});