module ActionDispatch::Http::Parameters::ClassMethods

ActionDispatch::Request.parameter_parsers = new_parsers
new_parsers = original_parsers.merge(xml: xml_parser)
xml_parser = -> (raw_post) { Hash.from_xml(raw_post) || {} }
original_parsers = ActionDispatch::Request.parameter_parsers

and the value is a proc.
It accepts a hash where the key is the symbol of the MIME type

Configure the parameter parser for a given MIME type.

def parameter_parsers=(parsers)
  @parameter_parsers = parsers.transform_keys { |key| key.respond_to?(:symbol) ? key.symbol : key }
end