this solves my headaches using pretty urls without extension and local dev server

the problem (v0.7.0): 
1. I have somefile.md in my jekyll project. 
2. I run webrick dev server locally
3. I do `curl -I http://localhost/somefile`
4. I get `Content-Type: application/octet-stream` because there is no content type definition and webrick forces this mime type

see also http://support.github.com/discussions/repos/4075-gh-pages-jekyll-permalink-does-not-work-content-type-applicationoctet-stream