Thread:Extension talk:MsUpload/Uncaught ReferenceError: wgFileExtensions is not defined - Mediawiki 1.25/reply

Since MediaWiki 1.25,  is only set on Special:Upload (181090).

The MsUpload extension has been updated to use the Siteinfo API instead (181107). If you're using the latest MediaWiki, the latest MsUpload should work.