hs-img-upload.js

/**
    Uploading files
    Taken from: http://mikejolley.com/2012/12/using-the-new-wordpress-3-5-media-uploader-in-plugins/

    INSTRUCTIONS:
    In the page you wish to use this script you'll need to enque the media scripts and this script.
    Then create a input box and a button with NOTHING in between.. with the class 'upload_image_button'

    <?php
        wp_enqueue_media();
        wp_enqueue_script( $handle = 'hs-img-uploader',
                           $src = get_bloginfo('template_url') . '/admin/js/hs-img-upload.js',
                           $deps = array('jquery'),
                           $ver = false,
                           $in_footer = true );
    ?>

    ...

    <input type="text" class="regular-text" id="Thumb" name="Thumb"><button class="button-secondary upload_image_button">Browse...</button>


**/


//File Upload vars
var file_frame;
var wp_media_post_id = (wp.media == undefined) ? null : wp.media.model.settings.post.id; // Store the old id
var set_to_post_id = 2; // Set this (from experminentation has to be a valid ID)
var jButton = null;

jQuery(document).ready(function(){
    jQuery('.upload_image_button').live('click', function( event ){
        event.preventDefault();
        jButton = jQuery(this);

        // If the media frame already exists, reopen it.
        if ( file_frame ) {
            // Set the post ID to what we want
            file_frame.uploader.uploader.param( 'post_id', set_to_post_id );
            // Open frame
            file_frame.open();
            return;
        } else {
            // Set the wp.media post id so the uploader grabs the ID we want when initialised
            wp.media.model.settings.post.id = set_to_post_id;
        }

        // Create the media frame.
        file_frame = wp.media.frames.file_frame = wp.media({
            title: jQuery( this ).data( 'uploader_title' ),
            button: {
                text: jQuery( this ).data( 'uploader_button_text' ),
            },
            multiple: false  // Set to true to allow multiple files to be selected
        });

        // When an image is selected, run a callback.
        file_frame.on( 'select', function() {
            // We set multiple to false so only get one image from the uploader
            attachment = file_frame.state().get('selection').first().toJSON();

            // Do something with attachment.id and/or attachment.url here
            jButton.prev('input[type="text"]').val(attachment.url);

            jButton = null;

            // Restore the main post ID
            wp.media.model.settings.post.id = wp_media_post_id;
        });

        // Finally, open the modal
        file_frame.open();
    });

    // Restore the main ID when the add media button is pressed
    jQuery('a.add_media').on('click', function() {
        wp.media.model.settings.post.id = wp_media_post_id;
    });
});