[Osiris]

If there is one thing for sure it is that spammers are creative in finding new ways to bypass anti spam filters. They send distorted pictures, add random text to their messages, zip their spam, use excel files, pdf files and nowadays fdf files. Fdf files ? What’s that again ?
Adobe Fdf is the file extension for the Forms Data Format which I’m sure neither you nor me have ever heard before. The real beauty of the format for spammers is that the Adobe Reader is able to open and display the contents without problem. A great way for spammers to provide everyone with up to date news of stock market insider tips without having to rely on camouflage tactics to bypass the spam filters.
The best way to handle this sort of spam is to create a new spam filter that would move mails with an .fdf attachment right into the junk folder. No company that I know of is sending .fdf files to their customers which means that it is 100% save to automatically move them to the junk folder.