Seems to be a regression of an issue fixed in the past, see FE-727. Same steps to reproduce:
1) Navigate to https://extranet.atlassian.com/crucible/browse/FE/trunk/test/testfiles/simple or to some directory within your repository
2) In the Tarball box, click on zip and download the zip
3) Double click on zip file and you will get the error "The Compressed (zipped) Folder is invalid or corrupted"

Apparently FishEye is serving zipped file which gets gzipped by Jetty filter and MSIE8 (and MSIE7 but not MSIE9) struggle to open or save this file properly. See http://support.microsoft.com/kb/2002350

This issue is fixed in recent Jetty versions: http://jira.codehaus.org/browse/JETTY-1409 .

In the meantime we can change the Content-Type header so MSIE8 would be able to cope with it.