Ok, I'm still having issue finding the correct way to support https://github.com/symfony/symfony/pull/8944/files#L11R131 using xml.

@docteurklein rose up the fact that it might not be possible (https://github.com/gquemener/symfony/commit/b41388e6336ab9a8dffd105ff8b37f8792391732#commitcomment-4065417), but the FrameworkExtensionTest architecture suggest that all configuration format should behave identically. Is it okay if it's not the case?

I mean if it's not possible to define multiple mime-types for a given format by using yaml.