Manual:$wgCopyUploadsFromSpecialUpload/de-formal

Details
By default, upload by URL, when enabled via, is only possible using the API (or extensions such as UploadWizard). To make the option usable from Special:Upload, you can set this variable to true.

In order to make use of this option, users also need to have the option  set to true and the associated user right.

Example usage
This example setup will allow all autoconfirmed users to upload by URL, via Special:Upload or the API. Add to the bottom of your :