Sebastián,
The file name is actually not 2061-omnet-40-source--ide--tgz, but
omnetpp-4.0-src.tgz -- just paste the link into a browser, and you'll see!
The URL contains a redirect. The URL is an artifact of the DocMan plugin of
Joomla that we're using to manage downloads on the site. We have little
control over how DocMan forms URLs... I guess the program that you (or
gentoo) use to download the package tries to (incorrectly) deduce the
filename from the URL, instead of getting it from the HTTP Reply header or
whatever.
Regards
Andras