That means you knew in advance that it was coming. Even if the e-mail comes from a friend or colleage, do not open it. Don't open attachments with no extension at all. Often, attachments sent from a Mac have no extension, so you cannot tell the type of file. On a PC, by default, Windows does not show extensions.
File extensions provide information about the type of file.
Change this setting so you can see file extensions. Open Windows Explorer or My Computer. If there is a View tab, click it. If there is not View tab, click the File Types tab. Click OK to close the dialog box.