Open an email which has got an attachment.
At the bottom of the window it will say "1 attachment" followed by the attachment's name.
Single left-click on that name and a small box will open.
The bottom line of this box says "Do this automatically for files like this from now on" - single left-click the tiny box to the left of this writing to select it.
Single left-click the "OK" button.
Next time you get an attachment of this type it will open automatically when you single left-click its name.