Saving file to drupal site using drupalgap
Once you have the File ID, that means you've successfully saved the file to your Drupal site. Next up (typically), you'll want to attach that File ID to a new/existing Entity (e.g. node).

If you are creating a new node, use a POST, if updating an existing one, use a PUT.

New


  /my_endpoint/node.json


{
  type: 'article',
  language: 'und',
  title: 'Hello',
  file_images: {
    und: [ {fid: 12 }]
  }


}

Existing


  /my_endpoint/node/456.json


{
  type: 'article',
  language: 'und',
  title: 'Hello Again',
  file_images: {
    und: [ {fid: 12 }]
  }


}

FYI, the latest version of DrupalGap now supports image fields, so you shouldn't have to write any custom code.