ate_attachment_metadata( $attach_id, $file[‘name’] );
wp_update_attachment_metadata( $attach_id, $attach_data );
This code is used to upload a file (e.g. myfile.txt) as an attachment to the post with the ID of 123. The code first declares a file array with the file details, such as its name, type, temporary name, error code and size. Then, the wp_insert_attachment() function is used to add the file as an attachment to the post with the ID of 123. If the wp_generate_attachment_metadata() function does not exist, it is loaded from the WordPress includes/image.php file. Finally, the attachment metadata is generated and updated for the post.