Possibility to change FileAdder class in config in order to add custom logic?

[mrvnklm]

I would like to extend the FileAdder as I am currently working with some files that have expiry. Having withExpiry(DateTime $expiry) would be super handy. I am currently using the custom properties but as my media table grows and I am dealing with a lot of custom properties I find it to be the wrong place.

Would it be possible to add the FileAdder class to config so everybody can extend it to their needs? Would create a PR if its likely to be merged.