Bug 878042 - [MMS] image placeholder style, r=julienw
• apply the thumbnail style specs that have been defined by the UX team;
• for image attachments, generate a real thumbnail of the attachment rather
  than relying on a CSS background -- this allows to revoke the Object URL
  early, so we can get rid of a few hacks (= the <img> in the frame template
  and the setTimeout() in the onload listener);
• display the attachment size with an appropriate unit (KB or MB).
Merge pull request #10268 from fabi1cazenave/mmsThumbnails-bug876467

Bug 878042 - [MMS] image placeholder style, r=julienw