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
Okay. Does it make sense then ? The IO handler knows what it can use (buffer or path), and we would only inject this service into handlers that would use it (flysystem would not, since it has its own API).