Manual:$wgImgAuthDetails

Details
This configuration variable is only applicable if you are using Image Authorization.

For security reasons, you usually don't want your users to know why access was denied, just that it was. When set to false (default), the denial screen is generic.

If you want to change this, you can set $wgImgAuthDetails to 'true' and MediaWiki will give the user the reason why access was denied.