Feature summary
Requesting img_auth.php in an unauthenticated state, should start the "auto-login" functionality of Extension:PluggableAuth, just like index.php does.

Use case(s)
When users send links to uploaded files via messenger or mail, the recipient does not enter the wiki via index.php. In case the recipient is not authenticated he will only see an error message. He'd need to manually log into the wiki first and then re-loaf the URL to the uploaded file. This is very cumbersome.

Benefits (why should this be implemented?):
A user can directly access an uploaded file via URL without having to log into the wiki first.