Recently, we reported how HTML attachments were being used in various spam campaigns such as phishing attacks, email harvesting attacks, and 419 scams. Spammers have included a few more file formats, again in an attempt to escape anti-spam filters. As experienced previously with HTML attachments, these new file formats are also getting used in several different spam categories.
Spammers Exploring New File Formats: http://www.symantec.com/connect/blogs/spammers-exploring-new-file-formats.