Ready only permitions on Serv-U server

Hi, my name is Rui and I need help on Serv_U server.

All my users access at the same time files with FTP Voyager with the permissions that we created but when someone try to modify the application don´t prompt warning inform that the file are open.

This issue cause problem because on the same file all the users can ready or write…on the same time and we need to know if

is possible the Serv-U prompt and denied if the file is open for the first user opened the file and the others can´t access to write that file.

Thanks.