Hi nikk,
go to administrator/components/com_abook/helpers/files.php the modify line 25 adding the extension you need
from:
public static function files( $name, $active = NULL, $javascript = NULL, $directory, $extensions = "pdf|doc|odt|txt|epub|xls"
to:
public static function files( $name, $active = NULL, $javascript = NULL, $directory, $extensions = "pdf|doc|odt|txt|epub|xls|fb2|zip"