Loading animations via web request

Hi guys, I'm pretty stuck at the moment, it would be great if you could help me out.

I'm trying to download animations in the shape of Monkey Motion (.mm) files during runtime via Communications.makeWebRequest() from my REST API.

The error code I'm getting is -1002 - UNSUPPORTED_CONTENT_TYPE_IN_RESPONSE.

Here's the watch code:

function requestAnimation() {
var parameters = {
Contract.VideoServerData.PARAMETER_GARMIN_ANIMATIONS_ID => "dummy"
};
  var options = {
   :method => Communications.HTTP_REQUEST_METHOD_GET,
    :headers => {
     Contract.VideoServerData.PARAMETER_GARMIN_ANIMATIONS_ID => "dummy"
    },
    :responseType => Communications.HTTP_RESPONSE_CONTENT_TYPE_ANIMATION
  };
  Communications.makeWebRequest(
Contract.VideoServerData.URL_GARMIN_ANIMATIONS,
parameters,
options,
method(:processAnimation)
);
}
function processAnimation(responseCode, data) {
System.println("responseCode: " + responseCode);
if (responseCode == 200) {
   Application.Storage.setValue("dummy", data);
  }
}

The API was tested and seems to work fine. It returns a .mm file as Content-Type 'image/vnd.garmin.connectiq.animation' from an express.js server. Here's the regarding snippet:

app.get('/api/v2/animations_garmin', function(req, res){
let fileId = req.get("animation-id")
const file = path.join(__dirname, './data/animationsGarmin/' + fileId + '.mm')
res.type('image/vnd.garmin.connectiq.animation')
res.sendFile(file)
})

here's a result header:

HTTP/1.1 200 OK
Accept-Ranges: bytes
Cache-Control: public, max-age=0
Content-Length: 162984
Content-Type: image/vnd.garmin.connectiq.animation
Date: Wed, 06 Jan 2021 20:04:03 GMT
Etag: W/"27ca8-176d93d9d74"
Last-Modified: Wed, 06 Jan 2021 19:47:42 GMT
Server: Caddy
X
-Powered-By: Express

I'm testing both physical (Venu) und via simulator (Fenix 6 S Pro).

Please let me know if you have any idea where I'm mistaking.