title: Get Public URL from file uploaded with firebase-admin
body: <p>I use firebase-admin and firebase-functions to upload a file in Firebase Storage.</p>
  
  <p>I have this rules in storage:</p>
  
  <pre><code>service firebase.storage {
    match /b/{bucket}/o {
      match /images {
        allow read;
        allow write: if false;
      }
    }
  }
  </code></pre>
  
  <p>And I want get a public URL with this code:</p>
  
  <pre><code>const config = functions.config().firebase;
  const firebase = admin.initializeApp(config);
  const bucketRef = firebase.storage();
  
  server.post('/upload', async (req, res) =&gt; {
  
    // UPLOAD FILE
  
    await stream.on('finish', async () =&gt; {
          const fileUrl = bucketRef
            .child(`images/${fileName}`)
            .getDownloadUrl()
            .getResult();
          return res.status(200).send(fileUrl);
        });
  });
  </code></pre>
  
  <p>But I have this error <code>.child is not a function</code>.
  How can I get the public url of a file with firebase-admin?</p>