Custom File Extensions in Javascript

I noticed there a few threads about this around 2011, but nothing specifically how to do it in JavaScript. For Max, you can find the info under fileformat on the Controlling Max with Messages documentation page. Short answer though is to make a message box with the following....

; max fileformat .customFileExtension desiredFileType 

Then bang or click on the message to send it to Max and associate the new extension with the filetype. Based on the example on the bottom of the js Max Object page and some testing of other methods and properties, this seems like it would be the equivalent syntax...

this.max.fileformat(".customExtenstionNameHere", "desiredFileType");

However, so far no luck. What am I missing here?

In my specific case, I am trying to use the Dict object method .import_json() to import a json file with a custom extension. I am use a copy of a .json file that works great until the custom extension is used.