Media type, e.g. application/pdf. Parameters are dropped when stored.
OptionalfilenameStored with the record. Sent as X-Filename when it is printable ASCII,
otherwise as Content-Disposition: attachment; filename*=UTF-8''...,
which the Personal Server decodes back to the same string. Must not have
leading or trailing whitespace (HTTP would strip it and the signed
representation would no longer match).
Bytes to store as an unstructured (binary) record.