Metadata extractors for the file abstraction layer (FAL) 

Metadata extractors are service classes that are automatically executed whenever an asset / file is added to FAL storage or when FAL indexing is executed.

If \TYPO3\CMS\Core\Resource\Index\ExtractorInterface is implemented by a class, registration of Metadata extractors happens automatically. This is due to autoconfigure tagging by the Symfony Dependence Injection framework.