my $rows = $args{'rows'};
my %opt = %{ $args{'opt'} };
-my $format = $FS::CurrentUser::CurrentUser->spreadsheet_format;
+my $override = scalar(@$rows) >= 65536 ? 'XLSX' : '';
+
+my $format = $FS::CurrentUser::CurrentUser->spreadsheet_format($override);
+
my $filename = $opt{'name'} || PL($opt{'name_singular'});
$filename .= $format->{extension};