Without trailing dot:

```
~/dev/spider/pws/spiders ❯ ~/dev/scrapy/bin/scrapy crawl pws_spider.py -o ./data                                                      
Usage
=====
  scrapy crawl [options] <spider>

crawl: error: Invalid/unrecognized output format: , Expected ['xml', 'jsonlines', 'json', 'csv', 'pickle', 'marshal']
```
Result of `splitext` of `./data` will be empty file ext.
```
>>> import os
>>> fname = "./data"
>>> print os.path.splitext(fname)
('./data', '')
```