WordPress 3.5 custom media upload for your theme options

Your going about it in a way that was unintended. Your javascript code should probably look something like this:

$('.custom_media_upload').click(function(e) {    e.preventDefault();    var custom_uploader = wp.media({        title: 'Custom Title',        button: {            text: 'Custom Button Text'        },        multiple: false  // Set this to true to allow multiple files to be selected    })    .on('select', function() {        var attachment = custom_uploader.state().get('selection').first().toJSON();        $('.custom_media_image').attr('src', attachment.url);        $('.custom_media_url').val(attachment.url);        $('.custom_media_id').val(attachment.id);    })    .open();});

Don't forget to use wp_enqueue_media (new in 3.5) if you'r not on the post edit page

To use the old media upload box you can do this:

if(function_exists( 'wp_enqueue_media' )){    wp_enqueue_media();}else{    wp_enqueue_style('thickbox');    wp_enqueue_script('media-upload');    wp_enqueue_script('thickbox');}

I modified this code a bit more to make it usable for multiple fields at once:

HTML:

<!-- Image Thumbnail --><img class="custom_media_image" src="" style="max-width:100px; float:left; margin: 0px     10px 0px 0px; display:inline-block;" /><!-- Upload button and text field --><input class="custom_media_url" id="" type="text" name="" value="" style="margin-bottom:10px; clear:right;"><a href="#" class="button custom_media_upload">Upload</a>

jQuery:

jQuery(document).ready(function($){$('.custom_media_upload').click(function() {        var send_attachment_bkp = wp.media.editor.send.attachment;        var button = $(this);        wp.media.editor.send.attachment = function(props, attachment) {            $(button).prev().prev().attr('src', attachment.url);            $(button).prev().val(attachment.url);            wp.media.editor.send.attachment = send_attachment_bkp;        }        wp.media.editor.open(button);        return false;           });});