Get the type/subtype associated to a file extension ext

Property definitions

nitcorn $ MediaTypes :: []
	# Get the type/subtype associated to a file extension `ext`
	fun [](ext: String): nullable String
	do
		if types.keys.has(ext) then return types[ext]
		return null
	end
lib/nitcorn/media_types.nit:27,2--32,4