Fallback to content-type name if no cd/filename

* src/mimeparser.c (t2body): Fallback to ct/name if cd/filename
  not found or not parsed.

This is neccessary for enigmail attachments which uses an
rfc 2038 (i think) content-dispositon filename but an
RFC 2047 content-type / name so that Outlook can understand
the filename from the content-type / name. We understand this
now, too.