I have none of the appropriate ports forwarded, so I assume it does not need it. However, this document might be of use to you:
http://support.microsoft.com/default...;EN-US;q278887
Also, in the 'Filters' tab, you might try enabling the following:
Multicast Passthrough
IPSec Passthrough
PPTP Passthrough
None of those should have any effect on file transfer, but whatever, instant messenging protocols aren't known to be the smartest of thing. If this doesn't work, then I have no idea what the problem could be. Might just be a fluke, since I can't file-transfer with ICQ or AIM, but hey, good luck to you. :)
Also: It might be noted that the new firmware includes a number of fixes to be had. One good thing at Linksys is that they do at least try and keep up to date. Again, good luck.