即使我的代码要求POST，jQuery也会发送GET

Question

$("#yes-link").click(function () {
    $.ajax({
        type: "POST",
        url: "http://0.0.0.0:3001/api/helpful/",
        data: {url: "sad", value: "yes"},
        dataType: "jsonp",
        success: function(data) { alert(data); }
    });
 });

由于某种原因，服务器总是看到GET。

Started GET "/api/helpful/?callback=jQuery11110520667711738497_1405634368646&url=sad&value=yes&_=1405634368649" for 127.0.0.1 at 2014-07-17 18:01:30 -0400
Processing by Api::HelpfulController#get as JSON
Parameters: {
                "callback" = > "jQuery11110520667711738497_1405634368646",
                "url" = > "sad", 
                "value" = > "yes", 
                "_" = > "1405634368649"
                }
  User Load (0.2ms)  SELECT  "users".* FROM "users"  WHERE "users"."ip" = '127.0.0.1' LIMIT 1
  Helpful Load (0.1ms)  SELECT  "helpfuls".* FROM "helpfuls"  WHERE "helpfuls"."user_id" = 1 AND "helpfuls"."url" = 'sad' LIMIT 1
Completed 200 OK in 2ms (Views: 0.1ms | ActiveRecord: 0.3ms

此外，参数列表中的"_"=>"1405634368649"是什么？