rename Checksum-FileSize to Filesize in hashsum mismatch

rename Checksum-FileSize to Filesize in hashsum mismatch

Some people do not recognize the field value with such an arcane name
and/or expect it to refer to something different (e.g. #839257).
We can't just rename it internally as its an avoidance strategy as such
fieldname existed previously with less clear semantics, but we can spare
the general public from this implementation detail.