Previously a $defaultMapping protected property existed on \Drupal\Core\File\MimeType\ExtensionMimeTypeGuesser. However, services that just needed to change the map of mimetype to extensions (and visa versa) had to implement a whole alternative guesser.
The following methods on Drupal\Core\File\MimeType\ExtensionMimeTypeGuesser are now deprecated:
setMapping()getMapping()
The map is now provided by a separate Drupal\Core\File\MimeType\MimeTypeMapInterface property, with a Drupal\Core\File\MimeType\DefaultMimeTypeMap implementation and service. This allows different implementations to override the map service rather than the whole guesser service.
The protected $defaultMapping property on ExtensionMimeTypeGuesser has been removed. Users wanting to replace the mapping should inject their own implementation of Drupal\Core\File\MimeType\MimeTypeMapInterface instead.
The hook_file_mimetype_mapping_alter() hook is also deprecated and will be removed in Drupal 12. Users should implement an Drupal\Core\File\Event\MimeTypeMapLoadedEvent listener to alter the map instead.
Notice:
Review https://www.drupal.org/project/drupal/issues/3530461 regarding potential removal of the $fileSystem argument of ExtensionMimeTypeGuesser::__construct() in Drupal 11.3.0.
