Available from version 1.23.0 (Gerrit change 78926)
Allows including additional file metadata information in the extmetadata field of the imageinfo API
Define function:
public static function onGetExtendedMetadata( array &$combinedMeta, File $file, RequestContext $context, bool $single, int &$maxCacheTime ) { ... }
Attach hook:

In extension.json:

	"Hooks": {
		"GetExtendedMetadata": "MyExtensionHooks::onGetExtendedMetadata"

For MediaWiki ≤1.25:

$wgHooks['GetExtendedMetadata'][] = 'MyExtensionHooks::onGetExtendedMetadata';
Called from:File(s): media/FormatMetadata.php

For more information about attaching hooks, see Manual:Hooks.
For examples of extensions using this hook, see Category:GetExtendedMetadata extensions.