我已经在附件上启用了类别，我正在尝试实现自动处理“未分类”类别的逻辑，如下所示：

1） 如果附件没有类别，请将其设置为“未分类”

2） 如果附件除了“未分类”之外还有其他类别，请从帖子的类别列表中删除“未分类”

我已经处理了大部分逻辑，但现在我不知道如何从附件中删除类别。这是我的代码：

<?php

// Enable categories with attachments
register_taxonomy_for_object_type( \'category\', \'attachment\' );

if( is_admin() ) {

    add_action(\'add_attachment\', \'emt_set_attachment_category\');
    add_action(\'edit_attachment\', \'emt_set_attachment_category\');
    function emt_set_attachment_category( $post_ID ) {

        $attachmentCategories =  wp_get_object_terms( $post_ID, \'category\' );

        // if attachment has many categories, remove the default category
        if (count($attachmentCategories) > 1) {
            foreach ($attachmentCategories as $key => $category)
                if ($category->name == "Uncategorized")
                    // How to delete it??

            return;
        }

        // if attachment already have categories, stop here
        if (count($attachmentCategories) == 1)
            return;

        // if attachment has no categories, set default category
        wp_set_post_categories( $post_ID, array( get_option(\'default_category\') ) );
    }
}