FTP 8021 Port query

Hi All,
we are planning to use webemthods FTP Port 8021 to trigger our Integration.we will be placing the input file in ns folder of the service to be triggered.
My question is regarding ,when the service will be triggered.Is it possible that the Integartion can kick off while the file is still getting ftp ed? or will the Integration be kicked off only when the whole file is ftp ed completely??
we dont want to process an incomplete file.Thats our intention.

Can somebody pls throw some light on this??