EZP-23390: IOService loads whole file to get mime type with FS cluster handler

https://jira.ez.no/browse/EZP-23390

Todo:
- [ ] Get [legacy PR](https://github.com/ezsystems/ezpublish-legacy/pull/1082) merged and change composer.json to reflect that before merging this
Actually not necessary, as the mimeType is still on ValueObject built by loadBinaryFile()
As long as it is there (untill 6.0) it is the most effective way to get it, but I'll change this anyway in case someone uses this method in the '"edge" case (I'm assuming this dual service stuff is no longer needed in 6.0).