Board Thread:Support Requests - Getting Technical/@comment-24789339-20140722122707/@comment-4018164-20140722152258

MediaWiki:Common.js is for wiki-wide scripts that will affect everyone, whereas User:DekkenMinus/wikia.js or w:User:DekkenMinus/global.js affect yourself only. This means that if you're looking for a tool that will allow all users in the wiki to auto-license their files, you would put it in MediaWiki:Common.js. If you want a quick way to license files, but that feature in general (or any other script) isn't very relevant to a particular wiki, you should go for a personal JS page of yours.

P.S. if you're uploading multiple files using Special:MultipleUpload, the license that you give will affect all of them, so you might want to use that too for a quick upload.