Remove duplicate file extensions from output messages

If a file type was added to $wgFileExtensions by both local configuration
and defaults in an extension (eg TimedMediaHandler and LocalSettings.php
both adding 'ogg' and 'ogv') it was being listed twice in the UI messages
listing acceptable types.

Runs array_unique() over the array on various outputs.

Bug: 54378
Change-Id: I14cd098d8b27099f8f803630535f33549740295c
Merge "Remove duplicate file extensions from output messages"