function media_upload_default( $post_ID ) {
    $args = array( 
                    'ID'           => $post_ID, 
                    'post_title'   => '', 
                    'post_excerpt' => '', 
                    'post_content' => '', 
            );

    wp_update_post( $args );

}
add_action( 'add_attachment', 'media_upload_default' );

下記サイトを参考にしました。

How can I add a default description to uploaded files?
http://wordpress.stackexchange.com/questions/121709/how-can-i-add-a-default-description-to-uploaded-files