Don't know if this could have been written any better, guess it can as
this is my first "deep" look into Capybara. The problem is found in the
Capybara::Driver::RackTest; it doesn't forward the content type of the
file being uplloaded to Rack::Test::UploadedFile.new. UploadFile's
initializer sets the content type to text/plain as a default.
This will make for instance Paperclip's attachment validation fail when
testing with Cucumber and Capybara even though we are uploading a type
which should pass.
