* correctly determine what are attachments from modest's perspective;

  ie. non-image/ parts, even with "content-disposition: inline" are
  considered attachments from modest's pov.

note; added modest_tny_mime_part_is_attachment_for_modest; it works
  and improves upon the current situation; anyway, it's important to
  keep an eye for any mails which fail our heuristics

pmo-trunk-r3980