Use image file preview for files with a thumbnail assinged

[amazeika]

Right now the details/preview templates (image vs file) get picked up by simply checking the file extension. Instead the JS app should also check if the file has a thumbnail assigned and use it as an image representation of the file in question. This is important with PDF files for example since connect may push a file preview to the actual PDF file.

See #149

See: https://secure.helpscout.net/conversation/1469744801/11247/