Adding check for collection so they display nicely in the gallery, inste...

...ad of looking like a default document

Now...
![screen shot 2014-08-08 at 1 27 07 pm](https://cloud.githubusercontent.com/assets/1599081/3860192/93765f5c-1f21-11e4-8593-a9bda82be009.png)

After PR...
![screen shot 2014-08-08 at 1 26 13 pm](https://cloud.githubusercontent.com/assets/1599081/3860197/9ba6f95c-1f21-11e4-86e7-3bcaa6f2310d.png)

If you just add `collection?` to Sufia::GenericFile::MimeTypes, then you could just check for `document.collection?`, right?