Bug 1026528 - [Messages] Corrupted attachments do not display correctly. r=julienw

* Removed redundant element that is used for "corrupted" case only as it's more styling than content;
* Corrupted class moved to ".attachment" container as it's more appropriate here.
Btw, we can do it other way (I'm fan of flex :)):
```
.attachment.corrupted .thumbnail-placeholder {
  display: flex;
}

.attachment.corrupted .thumbnail-placeholder:after {
  flex: 1;

  content: '';
  background: url('icons/corrupted.png') no-repeat center center / 82px;
}