Lately, it was discovered that there are recent malicious programs exploiting the default behavior of Windows operating systems which hides file extensions from the user. This behavior can be used to trick users into executing malicious code by making a file appear to be something it is not. The option is enabled by default, but a user may choose to disable this option in order to have file extensions displayed by Windows. Many worms transmitted via emails are known to exploit hidden file extensions. 

An example of a worm exploited the hidden file extension was the VBS/LoveLetter worm, discovered in May 2000 which contained an e-mail attachment named "LOVE-LETTER-FORYOU. TXT.vbs". Other malicious programs have since used similar naming schemes. Examples include VBS/OnTheFly (AnnaKournikova.jpg.vbs) - discovered in February 2001 VBS/Timofonica (TIMOFONICA.TXT.vbs) - discovered in June 2000 VBS/CoolNote (COOL_NOTEPAD_DEMO.TXT.vbs) - discovered in May 2000 Basically, any users might think the file sent via emails as attachments are safe as they appear as safe normal .txt, .mpg, .avi files, when actually the file has malicious script or is executable, i.e .vbs, .exe.