Spammers are adopting obscure file formats to get attachments passed security filters, Symantec researchers have warned.According to Symantec, spammers have taken to using two formats in particular, the electronic fax format, eFax, and the Internet Explorer web archive format, MHT, in a bid to sidestep aggressive filtering for better known file types.
http://news.techworld.com/security/121442/
http://www.networkworld.com/news/2009/081909-spammers-test-filters-with-obscure.html