Below is the basic example how delete_attachment action works.

function wpdocs_delete_attachment( $post_id ) {
	// Do something before attachment deleted

	// get attached file dir
	$file = get_attached_file( $post_id );

	// get file name of attachment to be deleted
	$file_name = wp_basename( $file );
}
add_action( 'delete_attachment', 'wpdocs_delete_attachment' );