MultipartRequest.swift

struct MultipartRequest {

    private let boundary = UUID().uuidString
    private var bodyString: String = ""
    private var body = Data()

    var request: URLRequest

    init(
        url: URL,
        params: [String: Any],
        imageFiles: [String: NewImage]?,
        documentFiles: [String: NewDocument]?
        ) {
        
        let userAgent = "\(Bundle.applicationName)/\(Bundle.applicationVersionNumber); iOS/\(UIDevice.current.systemVersion))"

        request = URLRequest(url: url)
        request.httpMethod = "POST"
        request.setValue("multipart/form-data; boundary=\(boundary)", forHTTPHeaderField: "Content-Type")
        request.setValue(userAgent, forHTTPHeaderField: "User-Agent")

        buildBodyString(forParams: params)

        if let images = imageFiles { addImageFiles(images) }
        if let documents = documentFiles { addDocumentFiles(documents) }

        body.append("\r\n--\(boundary)--\r\n".data(using: String.Encoding.utf8, allowLossyConversion: false)!)

        request.httpBody = body
    }

    private mutating func buildBodyString(forParams params: [String: Any])  {

        bodyString     += "--\(boundary)\n"
        bodyString     += "Content-Disposition: form-data; name=\"api_key\"\r\n\r\n"
        bodyString     += "\(RTAPIKey)\n"

        bodyString     += "--\(boundary)\n"
        bodyString     += "Content-Disposition: form-data; name=\"auth_key\"\r\n\r\n"
        bodyString     += "\(Preferences.authKey)\n"

        for (key, value) in params {
            bodyString     += "--\(boundary)\n"
            bodyString     += "Content-Disposition: form-data; name=\"\(key)\"\r\n\r\n"
            bodyString     += "\(value)\n"
        }

        body.append(bodyString.data(using: String.Encoding.utf8, allowLossyConversion: false)!)

        print(bodyString)
    }

    private mutating func addImageFiles(_ files: [String: NewImage]) {
        for (key, value) in files {
            body.append("--\(boundary)\n".data(using: String.Encoding.utf8)!)

            body.append("Content-Disposition: form-data; name=\"\(key)\"; filename=\"\(value.fileName)\"\n".data(using: String.Encoding.utf8)!)
            body.append("Content-Type: \(value.mimeType)\r\n\r\n".data(using: String.Encoding.utf8)!)

            let fileExtension = ImageFileExtension(rawValue: value.fileExtension)
            switch fileExtension! {
            case .PNG: body.append(UIImagePNGRepresentation(value.image.resized)!)
            case .JPEG, .JPG, .HEIC: body.append(UIImageJPEGRepresentation(value.image.resized, GlobalConstants.imageQuality)!)

            body.append("\r\n".data(using: String.Encoding.utf8)!)
            }
        }
    }

    private mutating func addDocumentFiles(_ files: [String: NewDocument]) {
        for (key, value) in files {
            body.append("--\(boundary)\n".data(using: String.Encoding.utf8)!)
            body.append("Content-Disposition: form-data; name=\"\(key))\"; filename=\"\(value.fileName)\"\n".data(using: String.Encoding.utf8)!)
            body.append("Content-Type: \(value.mimeType)\r\n\r\n".data(using: String.Encoding.utf8)!)

            body.append(try! Data(contentsOf: value.document.fileURL))

            body.append("\r\n".data(using: String.Encoding.utf8)!)
        }
    }
}

what classes are "NewDocument" and "NewImage"?