add_attachment
add_attachment is fired when an attachment is added. It passes the ID of the attachment.
This hook is an action which means that it primarily acts as an event trigger, instead of a content filter. This is a semantic difference, but it will help you to remember what this hook does if you use it like this: add_action('add_attachment', 'your_function');
This hook was introduced in WordPress 2.0, and will not work in earlier versions.