Using helpers in an attribute

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  appName: 'Ember Twiddle',
  filename: 'some.pdf'
});

helpers.rooturl.js

import Ember from 'ember';

export function rooturl(/*params, hash*/) {
  return '/myapp/';
}

export default Ember.Helper.helper(rooturl);

templates.application.hbs

<h1>Welcome to {{appName}}</h1>

<p><strong>rooturl:</strong> {{rooturl}}</p>
<p><strong>filename:</strong> {{filename}}</p>
<a href="{{concat (rooturl) filename}}">{{filename}}</a>

twiddle.json

{
  "version": "0.10.5",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "2.8.0",
    "ember-data": "2.8.0",
    "ember-template-compiler": "2.8.0",
    "ember-testing": "2.8.0"
  },
  "addons": {}
}