So for some reason, attach_file is not setting the content_type when uploading .ics files.

* https://github.com/brynary/rack-test/blob/280ff54f50d25dd70e2ec1c55049e5ef7de126f3/lib/rack/test/uploaded_file.rb
* https://github.com/jnicklas/capybara/issues/1236
* https://github.com/jnicklas/capybara/blob/master/lib/capybara/spec/session/attach_file_spec.rb#L30