Errors with css on Heroku (Rack::Lint::LintError at /)

[tommyblue]

I'm deploying Jekyll-bootstrap on Heroku cedar (it needs Gemfile instead of .gems) and the S3 server fails in serving bootstrap.min.css. The file can be opened by source view, but Firebug gives an error about the file (The file isn't text).
The same site deployed with pure rack (using Rack::TryStatic), works fine.
Using the official bootstrap.css instead of bootstrap.min.css provided by jekyll-bootstrap, the css sometimes is correctly loaded.
You can see the errors at http://smooth-waterfall-6329.herokuapp.com/
Heroku gives randomly this error:

Rack::Lint::LintError at /
No Content-Type header found
Ruby    /app/vendor/bundle/ruby/1.9.1/gems/rack-1.2.5/lib/rack/lint.rb: in assert, line 19
Web     GET smooth-waterfall-6329.herokuapp.com

I open this bug both to rack-jekyll and jekyll-bootstrap because I'm not sure where's the problem