Bug 870892 - [MMS] [UX] Thread list. The attachment icon is croppped - r=julienw

Attachment icon uses its own <span> element now that is only displayed in [data-last-message-type='mms'] threads
Body text is hidden for mms, so we don't need to fix the 'undefined' bug
(cherry picked from commit 267ec4ee2d3bb409584c489a0b105cb34eaf3821)