filter to chmod uploaded files

Hi!

I need to give new chmod rights to uploaded files.
All uploaded files and genereated thumbnails have permissions 600, so I need to apply the rights 640.

I want to realize this through a plugin:

function dg_chmod_upload( $ret )
{
    @ chmod( $ret['file'], 0640 );
    return $ret;
}
add_filter( 'wp_handle_upload', 'dg_chmod_upload' );

But this does not solve the problems with the generated thumbnails. Which filter do I have to use to get the path of a just generated thumbnail?