send an image to mastodon

I am currently working on a program to post images to Mastodon, but I have used the one in the post below as a reference, but
http://qiita.com/aryzae/items/8c16bc456588c1251f48#_reference-32d9e31807ff872c27f8
When I send it, I check the amount of data of 85406 bytes, but the reception is displayed as 0 bytes, and I am worried.
Normally, there should be some reception in json format...
I looked at the API list of my home GitHub (translated by Chrome), and I am having trouble solving it.

Thank you for your cooperation.

    US>func testToot(){
        let session = URLSession.shared
        let i = Item()
        let url = URL (string: "https://pawoo.net/api/v1/media")!
        let param: [String: String] = ["access_token": i.getItem(clum: "access_token") as!String]


        let image : Data = UIImageJPEGRepresentation (UIImage(named: "a.jpg")!, 0.8!as Data
        let boundary = "Boundary -\ (NSUUID().uuuidString)"
        var req = URLRequest(url:url)
        req.httpMethod="POST"
        req.addValue("application/json", for HTTPHeaderField:"Content-Type")
        req.addValue("multipart/form-data; boundary=\(boundary)" for HTTPHeaderField: "Content-Type")

        req.httpBody=createBody(_param:param,_key:"file", imageData:image,_boundary:boundary)

        let task = session.dataTask(with:req, completionHandler: {data, response, error in do {
            print(response!)
            }
        })
        task.resume()
    }


    func createBody(_param:[String:String],_key:String, imageData:Data,_boundary:String)->(Data){
        let file="a.jpg"
        let type="image/jpeg"
        varbody=Data();
            for(key,val)in_param{
                body.append("--`\(_boundary)\r\n")
                body.append("Content-Disposition: form-data; name=\(key)")
                body.append("\(val)\r\n")
            }
        body.append("--(_boundary)\r\n")
        body.append("Content-Disposition: form-data; name=\"\(_key)\"; filename=\"\(file)\"\r\n")
        body.append("Content-Type\(type)\r\n\r\n")
        body.append(imageData)
        body.append("r\n")
        body.append("----(_boundary)--\r\n")
        return body
    }
}
extension Data {
    mutating funcappend(_str:String){
        let data = Data (str.utf8)
        return self.append(data)
    }
}

Response Contents

 { URL: https://pawoo.net/api/v1/media } {status code:400, headers {
    "Content-Length" = 0;
    "Content-Type" = "text/html; charset=utf-8";
    Date="Fri, 12 May 2017 12:44:39 GMT";
    Vary = Origin;
    "x-request-id" = "aasf0bb-fb82-45c1-34d0-676ffc1aww15";
    "x-runtime" = "0.155211";
} }