The solved part is countinue the process by Email and there is still need to be soloved by watch folder. Here are some steps.
First, I set the email endpoint on "complete task" service(in process management catagory). Then configure the input parameter of the endpoint. The attachment parameter set to *.* and the mail from set to %SENDER%.
Second, the xfa form variable of process must have process fields and use the default render. So the process field data which are required to counitnue the process will be keeped.
Then it worked.
But when I check the "complete task" component(process management --> task notification catagory) used in "complete task" process. Some input parameters(Sender, attachment map) are required and can't be found if using watch folder. So there is still a problem by using watch folder.