Don't generate a preview from non-text parts, e.g. multipart/signed.
Don't generate a preview from non-text parts, e.g. multipart/signed.

Bug 714317.

* src/engine/rfc822/rfc822-message-data.vala (PreviewText::with_header):
  Check to see if the part is text/plain or text/html before generating a
  preview for it.

tshikaboom/geary,tshikaboom/geary,tshikaboom/geary