To test uploading a file without using fixture_file_upload, a posted
ActionDispatch::Http::UploadedFile should not be paramified (just like
Rack::Test::UploadedFile).
(Rack::Test::UploadedFile and ActionDispatch::Http::UploadedFile don't
share the same API, tempfile is not accessible on
Rack::Test::UploadedFile as discussed in
https://github.com/brynary/rack-test/issues/30)