title: Type of expression is ambiguous without more context in Alamofire.upload swift 3
body: <p>Updated Alamofire 4.0.0 does not mention how to put Httpmethod &amp; Httpheaders in upload with multipartFormData. That's why I google and found solution in that stackoverflow question. But the problem is I did same as that answer then got following error message and building is failed. Please help me how to solve it.</p>
  
  <blockquote>
    <p>Type of expression is ambiguous without more context</p>
  </blockquote>
  
  <p>Here is my coding:</p>
  
  <pre><code>let URL = try! URLRequest(url: Config.imageUploadURL, method: .post, headers: headers)
  
  Alamofire.upload(
      multipartFormData: { multipartFormData in
          multipartFormData.append(self.imageData, withName: "image", fileName: "file.png", mimeType: "image/png")
      },
      to: URL,
      encodingCompletion: { encodingResult in
          switch encodingResult {
          case .success(let upload, _, _):
              upload.responseJSON { response in
                  if((response.result.value) != nil) {
  
                  } else {
  
                  }
              }
          case .failure( _):
  
          }
      }
  )
  </code></pre>