Attachments count as a built-in post type which is extremely similar to Posts and Pages. The field you’re looking for is post_author.
If you have an attachment object:
$attachment->post_author
If you only have the attachment ID you can use
get_post_field( $attachment_id, 'post_author );
To get the actual Post the the attachment was originally uploaded to you can use:
$post->post_parent