File transfer: vars.filenames is undefined

I'm using a File Transfer activity to load all csv files in SFTP. Docs say all files present on the server will be stored in vars.filenames. Why it says it is undefined? 

1. Listing of files

   To list the files, select the File listing option from the Action field. Configuration of the remote server is the same as for receiving files. For more information, refer to this section.

   The List all files option, available when selecting the File listing action, allows you to store all files present on the server in the event variable vars.filenames wherein the file names are separated by \n characters.

There are files in the server:

Javascript code:

var filanamesArray = vars.filenames.split('\n');

for (var i = 0; i < filenamesArray.length; i++) {
var filename = filenamesArray[i];
logInfo("file: " + filename);
}