When we overwrite a checksumed file with a file without a checksum we
should remove the checksum from the server.
This is done by setting the column to empty.
Instead of returning
<oc:checksum>TYPE:CHECKSUM</oc:checksum>
Return
<oc:checksums>
<oc:checksum>TYPE:CHECKSUM</oc:checksum>
</oc:checksums>
This will allow us to expand in the future to multiple checksums.
Without doing just string concatenation.
And even for a single checksum it is now correct.
Introduce a new property "oc:fileid" to return the internal file id.
This is because the original "oc:id" property is a compound and it is
not possible to extract the real id without knowing the instance id. The
instance id is not available to external clients.