'value' => prop value, 'source' => 'name of hook' ).
$file: File object of file in question
$context: RequestContext (including language to use)
+$single: Only extract the current language; if false, the prop value should
+be in the metadata multi-language array format:
+mediawiki.org/wiki/Manual:File_metadata_handling#Multi-language_array_format
&$maxCacheTime: how long the results can be cached
'GetFullURL': Modify fully-qualified URLs used in redirects/export/offsite data.
something completely different, after the basic globals have been set up, but
before ordinary actions take place.
$output: $wgOut
-$article: $wgArticle
+$article: Article on which the action will be performed
$title: $wgTitle
$user: $wgUser
$request: $wgRequest