Mail attachments

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

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:

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