Only local images are allowed in forum topics

Problem/Motivation

It is not possible to upload images in forum topics. If a user links to an external image in a topic, it is not displayed and "Only local images are allowed." is shown in the red-crossed missing image icon's alt and title attributes.

Also, after clicking CKEditor's image icon, it says "URL (must be hosted on Drupal.org, upload with “Add a new file”)" but there is no file upload option.

Steps to reproduce

Create a new Forum topic, and try to upload an image or link to an external image, and see that you can't, or it fails.

Proposed resolution

Allow image uploading in Forum topics, or offer a suggestion on where to upload an image on drupal.org, and allow it to be referenced.

Remaining tasks

User interface changes

API changes

Data model changes