content-type isn't properly set when detection fails
When a content-type couldn't be detected by mimetypes.guess_type, 'None' is output instead of 'application/octet-stream' due to a small typo. The attached patch fixes the problem.
I would have forked, branched, fixed, and created a pull request, but my mercurial chops aren't up to snuff.
Thanks for the great package.