RFC2045, section 5.1 seems to indicate that arbitrary parameters can be
appended to a Content-Type header specification. Some email clients seem to
use the Content-Type header to specify an attachment without giving a
separate Content-Disposition header normally used to signify attachments.

This patch corrects attachment detection for piped emails to detect such
emails. The patch also correctly decodes filenames specified either in the
Content-Disposition or Content-Type headers using RFC5987, where the
filename can be encoded using an arbitrary character set (ie, not us-ascii).