Download and save a FIT file with makeWebRequest

Context

I am building a Garmin App for running and I would like to know the type of terrain I a m running on (asphalt, ground, paved etc...).
I have a backend querying OpenStreetMap to labelised my tracks as LatLng array with the type of terrain eg : [[45, 5, 'asphalt'], [45.1, 5, 'asphalt'], [45.2, 5, 'ground']].

I would like to store those terrain information in the watch to access it during the activity in offline mode.
First i thought i will store them in the Application.Storage system but the size allowed is very limited.

Then I looked at makeWebRequest in order to download and save data into the PersistedContent. I would like to download/import FIT files and not GPX because too few devices support GPX.

First to test, I created a simple FIT file on https://connect.garmin.com/modern/courses.

Then I exported and save the FIT file in my laptop. Then started a webserver serving this file.

Issue

When I try to execute makeWebRequest it succeed with 200 responseCode but the fit file seems to not be saved because there is nothing I can read with PersistedContent.getCourses().
Also the data field in the callback is type Lang.object with a field : _data: Resource and when I try to inspect it in debugger mode it stop the debugger with the issue : 
Kill app for update variable children
Failed to update variable children: Timeout

The code 

function onReceive(responseCode, data) {
    if (responseCode != 200) {
        // THE EXECUTION DOES NOT ENTER HERE
        System.println("data == null\nCode " + responseCode.toString() + "\n");
        return;
    }

    if (data == null) {
        // THE EXECUTION DOES NOT ENTER HERE
        System.println("data is null");
        return;
    }
    var a = PersistedContent.getCourses();
    var b = a.next();
    System.println(a == null); // print 'false'
    System.println(b == null); // print 'false'
    
}

function download() {
   var url = "http://192.168.1.50:8080/track.fit";
   var options = {
       :method => Communications.HTTP_REQUEST_METHOD_GET,
       :headers => {
               "Content-Type" => Communications.REQUEST_CONTENT_TYPE_URL_ENCODED},
       :responseType => Communications.HTTP_RESPONSE_CONTENT_TYPE_FIT
   };
   Communications.makeWebRequest(url, {}, options, method(:onReceive));
}

Is the fit file automatically saved in PersistedContent with the makeWebRequest call ?
I inspected the HTTP traffic with the simulator and the fit file is correctly fetched.

Thanks for your help