Below is a list of file extensions belonging to e-mail attachments that you should never open, regardless of where they came from, or who sent them. A malicious e-mail attachment is one of the most effective and easiest methods of infecting a computer. Always check the file extension of an e-mail attachment, because cyber-criminals can disguise an attachment as a Microsoft Word document, PDF document, or other legitimate documents. Attachments ending with ".zip", ".rar", ".7z", "r01", ".iso", and ".tar" are compressed files that should not be open if the sender is unknown to you, or you were not expecting the email message.