Attachments and Kerio Firewall

[Smir]

A Forum I am using daily just changed from PhpBB to SMF 1.0.5.

And because I am using Kerio Personal Firewall, I cannot see attachment files (including all avatars).
I figured out, that I can see them, when I disable the web-filtering option of the firewall.

But this only solves the problem for me and not for all other users with KPF.
So I installed SMF 1.0.5 on my local machine and tried to find a way, to show the attachments without changing the firewall settings.

The Download() function of the Display.php sets some headers, before outputting the file.
One of them is:
header('Set-Cookie:');
(Line 813)

When I comment that line out, everything works fine.
If you want, you can change that, too... I don't think this header has any usefull effect, because it does not specify a cookie.