In your panel code get the path to your extension like this:
gPanelPath = gCSInterface.getSystemPath(SystemPath.EXTENSION);
gCSInterface.evalScript("WhereAmI('" + gPanelPath + "');");
Then pick it up in your Photoshop JavaScript like this:
var myPanelPath = "";
function WhereAmI(inLocation) {
myPanelPath = inLocation;
}
And then change your File line to this:
desc176.putPath( idnull, new File( myPanelPath + "/assets/file.psd" ) );
The single quote and double quote I use above may not copy paste well so double check that if you get errors when trying to run. I didn't actually run the code above so hopefully I don't have a syntax error.