mp3 upload metabox not working

Hi
I followed the excellent tutorial on creating metaboxes here,
but having tried many times, I have the same problem:
the $_FILES[‘mp3_attachment’][‘name’] is always empty.

Can’t figure out why. Help appreciated ??

The code is below:

‘talk’ is a custom post type.

// Add custom upload field for the mp3s
function add_custom_meta_boxes() {

	// Define the custom mp3 attachment for Talk posts
	add_meta_box(
		'mp3_attachment',
		'MP3 File',
		'mp3_attachment',
		'talk',
		'normal'
	);
} // end add_custom_meta_boxes
add_action('add_meta_boxes', 'add_custom_meta_boxes');

// Create mp3 upload form
function mp3_attachment() {

	wp_nonce_field(plugin_basename(__FILE__), 'mp3_custom_attachment_nonce');

	$html = '<p class="description">';
	$html .= 'Upload your mp3 file here.';
	$html .= '</p>';
	$html .= '<input type="file" id="mp3_attachment" name="mp3_attachment" value="" />';

	echo $html;
} // end wp_custom_attachment

// Save mp3 upload to database
function save_mp3_custom_meta_data($id) {

	/* --- security verification --- */
	if(!wp_verify_nonce($_POST['mp3_custom_attachment_nonce'], plugin_basename(__FILE__))) {
		return $id;
	} // end if

	if(defined('DOING_AUTOSAVE') && DOING_AUTOSAVE) {
		return $id;
	} // end if

	if('page' == $_POST['post_type']) {
		if(!current_user_can('edit_page', $id)) {
			return $id;
		} // end if
	} else {
		if(!current_user_can('edit_page', $id)) {
			return $id;
		} // end if
	} // end if
	/* - end security verification - */

	// Make sure the file array isn't empty
	if(!empty($_FILES['mp3_attachment']['name'])) {

		// Setup the array of supported file types. In this case, it's just PDF.
		$supported_types = array('audio/mpeg');

		// Get the file type of the upload
		$arr_file_type = wp_check_filetype(basename($_FILES['mp3_attachment']['name']));
		$uploaded_type = $arr_file_type['type'];

		// Check if the type is supported. If not, throw an error.
		if(in_array($uploaded_type, $supported_types)) {

			// Use the WordPress API to upload the file
			$upload = wp_upload_bits($_FILES['mp3_attachment']['name'], null, file_get_contents($_FILES['mp3_attachment']['tmp_name']));

			if(isset($upload['error']) && $upload['error'] != 0) {
				wp_die('There was an error uploading your file. The error is: ' . $upload['error']);
			} else {
				add_post_meta($id, 'mp3_attachment', $upload);
				update_post_meta($id, 'mp3_attachment', $upload);
			} // end if/else

		} else {
			wp_die("The file type that you've uploaded is not an mp3.");
		} // end if/else

	} // end if

} // end save_custom_meta_data
add_action('save_post', 'save_mp3_custom_meta_data');