It seems like EMail spammers have tried every attachment under the sun lately, but here's something I got this morning - an .FDF file bundled in with Spam mail:

An .FDF file is a text file format used for data exported from .PDF form fields. They're usually smaller than PDF files, because they only contain form field data, not the entire form. The content in the attachment was just the usual garbage relating to the "latest hot stock picks".

Nice.