[FIXED JENKINS-13536] Using file parameters could cause build records to not load.

Solves the issue for newly created builds by not attempting to persist the original file upload record, which is useless after the build has run (the fileParameters dir in the build already archives them).
Does not help loading pre-fix builds in which the upload temp file has been deleted; for these, you must manually delete <file class="org.apache.commons.fileupload.disk.DiskFileItem" serialization="custom">…</file> from the build.xml.
(Did not find a straightforward way to just instruct XStream to ignore this bit of XML: it is easy to avoid serializing a field, but deserializing is different.)