Fix detection of MIME type in message_parse()

Be careful, content-type may contain tricky content like in the
following example so test the MIME type with startswith()

Content-Type: multipart/related;
  boundary="_004_3f1e4da175f349248b8d43cdeb9866f1AMSPR06MB343eurprd06pro_";
  type="text/html"
Merge pull request #286 from jbq/bugfix

[FIX] mail: detection of MIME type

When parsing the mail headers, the content-type may has a 'type' attribute for Multipart/Related  objects (rfc2387).
Previous check would match on attached files of type text instead of real `content-type: text/`.