Bug in file paging

When searching for a file that does not exist:

```
File "/home/danilo/.virtualenvs/strongtnc/lib/python2.7/site-packages/django/core/handlers/base.py", line 114, in get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "/home/danilo/.virtualenvs/strongtnc/lib/python2.7/site-packages/django/views/generic/base.py", line 69, in view
    return self.dispatch(request, *args, **kwargs)
  File "/home/danilo/.virtualenvs/strongtnc/lib/python2.7/site-packages/dajaxice/views.py", line 52, in dispatch
    response = function.call(request, **data)
  File "/home/danilo/.virtualenvs/strongtnc/lib/python2.7/site-packages/dajaxice/core/Dajaxice.py", line 18, in call
    return self.function(*args, **kwargs)
  File "/mnt/data/Projects/ba/strongTNC/apps/core/decorators.py", line 17, in __wrapper
    return func(request, *args, **kwargs)
  File "/mnt/data/Projects/ba/strongTNC/apps/front/ajax.py", line 77, in paging
    page_count = sp(page_size, filter_query, producer_args, conf.get('static_producer_args'))
  File "/mnt/data/Projects/ba/strongTNC/apps/filesystem/paging.py", line 24, in file_stat_producer
    count = File.filter(filter_query).count()
TypeError: count() takes exactly one argument (0 given)
```