您可以在上传表单提交后运行以下函数。假设您的上传表单如下所示：

<form action="your_action.php" method="post"
enctype="multipart/form-data">
    <input type="file" name="attachment" />
    <input type="hidden" name="post_id" value="<?php global $post; echo $post->ID; ?>" />
</form>

在上传处理部分：

<?php

$filename = $_FILES["file"]["attachment"];

$post_id = $_POST["post_id"];

$filetype = wp_check_filetype( basename( $filename ), null );

$wp_upload_dir = wp_upload_dir();

$attachment = array(
    'guid'           => $wp_upload_dir['url'] . '/' . basename( $filename ), 
    'post_mime_type' => $filetype['type'],
    'post_title'     => preg_replace( '/\.[^.]+$/', '', basename( $filename ) ),
    'post_content'   => '',
    'post_status'    => 'inherit'
);

$attachment_id = wp_insert_attachment( $attachment, $filename, $post_id );
require_once( ABSPATH . 'wp-admin/includes/image.php' );
$attach_data = wp_generate_attachment_metadata( $attachment_id, $filename );
wp_update_attachment_metadata( $attachment_id, $attach_data );