Mac to PC attachments

[KEVIN Lock]

A friend in Sydney using a PC on ADSL cannot see my attachments. 
They take the usual time to download at his end, but he cannot find 
the attachment.  jpegs, mp3s, quicktime and WMA files do not appear 
at his end.

Is this likely to be a firewall setting?   He receives such files 
from other PC friends and can view and play them.

Any ideas I can give him?

TIA

Kevin