Can't get post parameters by request.param(name: "xxxx")

[ListenXunInNight]

Here is my code

static func signIn(request: HTTPRequest, resp: HTTPResponse) {

    var pwd = request.param(name: "pwd")
    var email = request.param(name: "email")
    
    defer {
        resp.completed()
    }

    guard pwd != nil && email != nil else {
        resp.appendBody(string: Response.JSON(.lackParameters))
        return
    }
}

Here is my request

POST /account/signIn HTTP/1.1
Host: 192.168.118.63:8181
Content-Type: application/json
Cookie: CSRF-TOKEN=BF6E8C61-8D4C-4501-BBD1-BFC223F997BF; PerfectSession=11279782-E82C-43BE-B986-71EEF1FB84E7
Accept: */*
User-Agent: GRedHeart5/1.0 (com.shabi.GRedHeart5; build:1; iOS 10.3.3) Alamofire/4.8.0
Accept-Language: zh-Hans-CN;q=1.0
Content-Length: 41
Accept-Encoding: gzip;q=1.0, compress;q=0.5
Connection: keep-alive

{"pwd":"666666","email":"[email protected]"}

Here is post parameters in request.postParams data structure

▿ 1 element
    ▿ 0 : 2 elements
- .0 : "{\"pwd\":\"666666\",\"email\":\"[email protected]\"}"
- .1 : ""

why I can't get post parameters by this method

public extension HTTPRequest {
    
    /// Returns the first GET or POST parameter with the given name.
    /// Returns the supplied default value if the parameter was not found.
    public func param(name: String, defaultValue: String? = nil) -> String? {
        for p in self.queryParams
            where p.0 == name {
                return p.1
        }
        for p in self.postParams
            where p.0 == name {
                return p.1
        }
        return defaultValue
    }
}

Maybe I prefer ask you why design like this?