Fix email composing for attaches

Hi!

Current implementation of e-mail composing put all of the mail parts into one multipart/related part. As result Ms Outlook (and possible some other mail clients) doesn't show attachment.

The idea of this fix is put all inline attachments into multipart/related part and, the other ones are placed in multipart/mixed part (togeher with multipart/related part). I've tested both images and attachments (I used pdf) with Outlook 2010 - all works for me.