Passing a PartnerProperties parameter when starting a FileTransferActivity object via SSJS

Question

Within the Salesforce Marketing Cloud, I am attempting to run a series of Data Extract & File Transfer activities sequentially, all with a different filename (ex myFile_2018-01-01.csv, myFile_2018-01-02.csv, etc). I am trying to use SSJS to dynamically set the filename value (I'll be reading from a Data Extension to get the value to use) when starting the FileTransferActivity object (and also the DataExtractActivity object). I can start the the File Transfer via the Perform method, but I cannot figure out the syntax for passing the appropriate PartnerProperties parameter.

https://developer.salesforce.com/docs/atlas.en-us.mc-apis.meta/mc-apis/filetransferactivity.htm

I've tried a fair number of variations, all to no avail. All of my attempts error, as it doesn't like my syntax:

var prox = new Script.Util.WSProxy();
var action = "Start";
var props = {CustomerKey: "myFileTransferKey" ,
 PartnerProperties: {FileSpec: "myFileName_2018-01-01.csv"} };

var opts = {};
var data = prox.performItem("FileTransferActivity", props, action, opts);