Bug 885584 - [B2G][SMS/MMS] Thumbnails are invisible in the preview attachment with large files - r=julienw

* Sets max thumbnail generation size to 1.5 megs, which should be big enough for any mms image
* Removes a var self = this;
(cherry picked from commit b17b09bf8526be355e2e56a31942c6d9b9d3adab)