i_rina, I have only 1 user account, so i tried the safe mode option. On reboot, with shift key down, the computer wouldn't finish the boot, and stopped midway. The slider beneath the apple icon was about halfway across. I did a hold-the-button shutdown, and let it reboot normally, which it did. I again looked at the mail attachment right-menu options, and noticed the view-as-icon option still missing. This behavior seems to have begun after upgrading to High Sierra. Thanks for responding. I would be grateful for any other advice.