Our own importer is happy about the export result already, but MSO is
more picky, and mandates the correct content types for both
_xmlsignatures/origin.sigs and the individual signature streams.

With this, MSO can open the signed file again (while previously it just
declared the file corrupted), though it still declares the signature
invalid.

Change-Id: I199ad96bb91e7ce03fdf1f10f9500db4e05bb5c1