I'm not sure how to reproduce the problem, as I'm just on the receiving end of it, but it's very common for for the attachments to get doubly compressed (.gz.gz). I've explicitly commented upon it in bug 519692 and the user's response was they didn't do anything special. Also, if bugzilla is already compressing attachments, would it be possible for it to try to *automatically* compress the larger logs (maybe up to a second size limit) ? One more thing: text/x-log auto detection, could that path be just disabled ? I don't mind the color sequences in portage logs, but bugzilla's autodetection scheme mishandles them, IMHO.
I recall seeing this in the past, and it was fixed at some point.
