Ignore Content-Id for Cnt-Disp: attachment

* src/mimedataprovider.cpp (t2body): Ignore content-id.

This should hopefully cover all cases where attachments
were hidden although they should have been shown. Outlook
itself seems to make the difference based on Content-Disposition.

According to documentation there are also ATTACH_FLAGS set
in case an attachment is hidden due to MHTML embeds. But
modifying those did not have an effect.

• GnuPG-Bug-Id: T4161