我相信你需要使用set_post_thumbnail 要实际设置特色图像：

set_post_thumbnail($_POST[\'post_id\'], $attachment_id);

需要从media_handle_upload 像这样：

First we get the new image ID

$attach_id = media_handle_upload( $file_handler, $_POST[\'post_id\'] );

Than we prepare the content of that alt="text":

// Prepare the alt="title"
$image_title = get_post( $post->ID )->post_title;
// Sanitize the title:  remove hyphens, underscores & extra spaces:
$image_title = preg_replace( \'%\\s*[-_\\s]+\\s*%\', \' \',  $image_title );
// Sanitize the title:  capitalize first letter of every word (other letters lower case):
$image_title = ucwords( strtolower( $image_title ) );

And finaly we set the new image alt="text":

add_post_meta( $attach_id, \'_wp_attachment_image_alt\', $image_title );