The notification type variable in mail notification activity can be easily over ridden by following the below post:
There are only two possibilities of notification Type variable it can be either Process Flow summary or any User Defined message.
These are selected by using following values for NotificationType variable and these are: STATIC and DYNAMIC. When the value is provided as STATIC then a user defined message will be sent but along with that you also need to over ride message of mail notification with some value in context variable(Refer attached screenshot for more details). On the other hand when it is set to DYNAMIC then it will send the PROCESS FLOW SUMMARY in notification.
Also make sure that *PUT Context Var* activity is placed before the mail notification activity in Process Flow.