Fix EZP-23390: Legacy IO loads whole file to get mime type with FS clust...

...er handler

Todo:
- [ ] Get [legacy PR](https://github.com/ezsystems/ezpublish-legacy/pull/1082) merged and change composer.json to reflect that before merging this
Hmmm. We did it this way: https://github.com/ezsystems/ezpublish-kernel/blob/master/eZ/Publish/Core/FieldType/Image/ImageStorage.php#L156. The warner uses `trigger_error( $message, E_USER_DEPRECATED );`, and symfony handles it fine. I used a custom service to make it easier to mock it and assert the message.