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

[darwin]

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

[RJ]

I had the same problem. Turns out you can add a "nil" mapping to the mime_types like so:
mime_types.store nil, 'text/html'

As per: #279

[parkr]

Woohoo!! All set? I think #491 takes care of this, too :)