MultipartRequest for Uploading images and forms to a server using URLSession in Swift

MultipartRequest.swift

/// MultipartRequest by Dan Rosenstark
/// Thanks to tinyurl.com/2qpbyh7v: this code is just a remix
///
/// Steps to use
/// 1. init
/// 2. add form fields (if any)
/// 3. add file data (if any)
/// 4. grab the urlRequest and use it
struct MultipartRequest {
    private let boundary = "Boundary-\(UUID().uuidString)"
    private let url: URL

    private var formFields = [String]()
    private var fileDatas = [Data]()

    init(url: URL) {
        self.url = url
    }
    
    var urlRequest: URLRequest {
        var request = URLRequest(url: url)
        request.httpMethod = "POST"
        request.setValue("multipart/form-data; boundary=\(boundary)", forHTTPHeaderField: "Content-Type")
        request.httpBody = body
        return request
    }

    mutating func addFormField(named name: String, value: String) {
        var fieldString = "--\(boundary)\r\n"
        fieldString += "Content-Disposition: form-data; name=\"\(name)\"\r\n"
        fieldString += "\r\n"
        fieldString += "\(value)\r\n"
        formFields.append(fieldString)
    }
    
    mutating func addFileData(fieldName: String, fileName: String, mimeType: String, fileData: Data) {
        let data = NSMutableData()
        
        data.append(string: "--\(boundary)\r\n")
        data.append(string: "Content-Disposition: form-data; name=\"\(fieldName)\"; filename=\"\(fileName)\"\r\n")
        data.append(string: "Content-Type: \(mimeType)\r\n\r\n")
        data.append(fileData)
        data.append(string: "\r\n")
     
        let result: Data = data as Data
        fileDatas.append(result)
    }

    private var body: Data {
        let body = NSMutableData()

        for field in formFields {
            body.append(string: field)
        }
        
        for fileDatum in fileDatas {
            body.append(fileDatum)
        }

        body.append(string: "--\(boundary)--")
        return body as Data
    }
}

fileprivate extension NSMutableData {
  func append(string: String) {
    if let data = string.data(using: .utf8) {
      self.append(data)
    }
  }
}