models: take care of <mount>_type field outside of model class

The crux of the problem is that the <mount> field of these models are
handled by CarrierWave/GridFs. While the forms work to
create/update these models, the '<mount>_type' field is left alone by
these operations on the model. So, setting the '<mount>_type' from the
controller is far more sensible.
binary_file_view: fix binary group test

This is one of those rare Ruby gotchas, where double quotes have
different functionality than single quotes.
binary_file_view: do not always call thumbnail version

This is because some uploaders do not process a thumbnail version (e.g.,
FIleUploader does not, whereas ImageUploader does). We call this
function as a view helper on either of these data objects, so we have
take the lowest common denominator.
binary_file_view: put filename for binary file link