MultipartData model [limitation: supports 1 item only] Reference: https://www.w3.org/TR/html401/interact/forms.html#h-17.13.4.2

MultipartData.swift

import Foundation

class MultipartData {
  let data:Data
  let filename:String
  let key:String
  let mimeType:String
  private let boundary:String
  
  init(data:Data, key:String, filename:String, mimeType:String) {
    self.data = data
    self.key = key
    self.filename = filename
    self.mimeType = mimeType
    self.boundary = "\(arc4random())"
  }
  
  private var contentType:String {
    return "multipart/form-data; boundary=\(boundary)"
  }
  
  private lazy var dataValue:Data = {
    let _data = NSMutableData()
    _data.append("--\(boundary)\r\n".data(using: String.Encoding.utf8)!)
    _data.append("Content-Disposition: form-data; name=\"\(key)\"; filename=\"\(filename)\"\r\n".data(using: String.Encoding.utf8)!)
    _data.append("\r\n".data(using: String.Encoding.utf8)!)
    _data.append(self.data)
    _data.append("--\(boundary)--".data(using: String.Encoding.utf8)!)
    return _data as Data
  }()
  
  private var contentLength:String {
    return "\(dataValue.count)"
  }
  
  func asURLRequest(url:URL, httpMethod:String = "POST") -> URLRequest? {
    let _request = NSMutableURLRequest(url: url)
    _request.httpMethod = httpMethod
    _request.httpBody = dataValue
    _request.addValue(
      contentType,
      forHTTPHeaderField: "Content-Type")
    _request.addValue(
      "\(contentLength)",
      forHTTPHeaderField: "Content-Length")
    return _request as URLRequest
  }
}