Add an interface for FileUploadHandler

Nice, wasn't sure what the policy is nowadays with adding interfaces for everything, but this would allow us to decorate the file.upload_handler service again.

Looked at the MR, looks good, searched for other uses and this seems like everything, only some comments now reference the class directly, but those point to a protected method on the class so that seems correct.

One comment on the MR, fine to self RTBC

Addressed feedback

Committed to 11.x - thanks!

larowlan → closed merge request !10387