class ActionDispatch::ParamsParser

In case of any error while parsing data ParamsParser::ParseError is raised.
that is picked based on Content-Type header.
(like POST). It takes raw data from the request and puts it through the parser
ActionDispatch::ParamsParser works for all the requests having any Content-Length

def self.new(app, parsers = {})

content mime type, and value is a lambda that is going to process data.
The +parsers+ argument can take Hash of parsers where key is identifying

Create a new +ParamsParser+ middleware instance. 
def self.new(app, parsers = {})
  ActiveSupport::Deprecation.warn('ActionDispatch::ParamsParser is deprecated and will be removed in Rails 5.1. Configure the parameter parsing in ActionDispatch::Request.parameter_parsers.')
  parsers = parsers.transform_keys { |key| key.respond_to?(:symbol) ? key.symbol : key }
  ActionDispatch::Request.parameter_parsers = ActionDispatch::Request::DEFAULT_PARSERS.merge(parsers)
  app
end