Manual:$wgAllowCopyUploads

Details
Users also need to have the associated GroupPermissions>Special:MyLanguage/Manual:$wgGroupPermissions|user right  before this option is available. Prior to MediaWiki 1.20, administrators (sysops) had this permission by default, but since then the permission needs to be granted explicitly.

By default, upload by URL are only possible using the upload>Special:MyLanguage/API:Upload|API (or extensions such as ). To make the option usable from Special:Upload, you need to set   to true as well.

By default, any source URL location is allowed, but you can whitelist a specific set by using  .

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 :