Attachment preview is enabled by default in Outlook 2016, 2013 and 2010 but for some reason if preview option is missing then you can turn on by following steps.
Click the File menu –> Options link. In the Outlook options window, navigate to Trust Center.
Then navigate to Microsoft Outlook Trust Center section and click Trust Center Settings button.
In the Trust Center window, click the Attachment Handling option.
Now un mark the check box with label as Turn off Attachment Preview. By this way you can enable attachment preview in Outlook 2016, 2013 and 2010. You can now verify whether attachment preview is enabled by opening an email with an attachment and right clicking on the attachment. This would display the Preview option as part of the menu list.