6 files declare their use of MediaHelper in Lightning Media 8

BulkUploadForm.php in modules/lightning_media_bulk_upload/src/Form/BulkUploadForm.php

EntityFormProxy.php in src/Plugin/EntityBrowser/Widget/EntityFormProxy.php

FileUpload.php in src/Plugin/EntityBrowser/Widget/FileUpload.php

lightning_media.module in ./lightning_media.module

Core media asset support for Lightning.

MediaForm.php in src/Form/MediaForm.php

MediaImage.php in src/Plugin/entity_embed/EntityEmbedDisplay/MediaImage.php