Mail attachments

It will always try to save if you try to click the paper clip icon. The only way way to double-click an attachment and have it displayed is if the attachment is part of the email, as a displayed icon in the body of the email, which you can double-click:

Boot into Safe Mode according to Start up your Mac in safe mode - Apple Support and test to see if the problem persists. Reboot normally and test again.

NOTE: Safe Mode boot can take up to nearly twice as long as a normal boot as it's doing the following; 

• Verifies your startup disk and attempts to repair directory issues, if needed

• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)

• Prevents Startup Items and Login Items from opening automatically

• Disables user-installed fonts 

• Deletes font caches, kernel cache, and other system cache files

You are welcome! I know how confusing it was for myself as well ;)