I have a problem with downloading PDF file using NSURLSession with POST.My download url is different than any other else because it didn't end with file extension name and it also need parameters to download that file.

It's from ASPX base server and return url like that

http://example.com/DownloadPDF.aspx?File=ES2Ges%2BUA1CEe1150UF3uUgEDeCEMoIuOrJCbwjzVKCEfA6%2F1zKxycl6MMWcjvtk

If I have to access the pdf and download it,I have to provide 2 parameters ConfigKeyID and AccessToken.And one thing strange is it was POST request. Since,Alamofire only support GET request in downloading,so,i have to write my own self using NSURLSession.But,I have problem with accessing the file from my code because it return html page with 401.

Here is the html :

401 - Unauthorized: Access is denied due to invalid credentials.

Server Error

401 - Unauthorized: Access is denied due to invalid credentials.

You do not have permission to view this directory or page using the credentials that you supplied.

Here is my code

protocol PDFDownloadAPIProtocol{
    func didSuccessGettingPDF()
    func didFailGettingPDF(err:NSError)
}

class PDFDownloadAPI{

var delegate : InvoicePDFDownloadAPIProtocol

init(delegate: InvoicePDFDownloadAPIProtocol){
    self.delegate=delegate
}

func post(url:String,accessToken:String,configKey :String){
    let url:NSURL = NSURL(string: url)!

    print(url)
    let session = NSURLSession.sharedSession()

    let request = NSMutableURLRequest(URL: url)
    request.HTTPMethod = "POST"
    request.cachePolicy = NSURLRequestCachePolicy.ReloadIgnoringCacheData

    let param = "ConfigKeyID=\(configKey)&AccessToken=\(accessToken)"
    request.HTTPBody = param.dataUsingEncoding(NSUTF8StringEncoding)

    let task = session.downloadTaskWithRequest(request) {
        (
        let location, let response, let error) in

        guard let _:NSURL = location, let _:NSURLResponse = response  where error == nil else {
            print("Location Error")
            self.delegate.didFailGettingPDF(error!)
            return
        }

        let urlContents = try! NSString(contentsOfURL: location!, encoding: NSUTF8StringEncoding)

        guard let _:NSString = urlContents else {
            print("Content Error")
            self.delegate.didFailGettingPDF(error!)
            return
        }

        print("Downloaded Content : \(urlContents)")

        self.delegate.didSuccessGettingPDF()
    }

    task.resume()
}

func doDownloadData(url : String,accessToken:String,configKey : String){
    post(url,accessToken: accessToken,configKey : configKey)
}
}

When i tried with Postman everything work file and i can even download file from chrome.So,I think there is something wrong with my code.Any Help?

Chances are, authentication requires more than just POST request fields. It probably uses either cookies or authentication headers (based on actual credentials).

If I'm wrong, then I'd check the following:

Make sure those two magic values are properly URL-encoded prior to adding them into the POST body.
Make sure you set the Content-Type header so the server knows that you're providing the body in x-www-form-urlencoded format.
Sniff the request with Wireshark and see what is actually getting sent.
Check the server logs if you can.