sinatra/sinatra — pull request #714 by rakoo (2013-05-19 12:48:33+00:00)
Title: Use Rack::File's Content-Type in send_file

When doing multipart, file.serving(env) will set the Content-Type with
the boundary, so we have to import it.

This is dependent of [multiple byte-request serving in sinatra](https://github.com/rack/rack/pull/559), not yet merged.