Using MultipartRequest to upload file

I'm using Goa v3 to design an endpoint that allows me to upload files (more precisely, images) with a multipart/form-data POST request. I have declared the following Service:

var _ = Service("images", func() {
    HTTP(func() {

    Method("upload", func() {  
        HTTP(func() {

        Payload(func() {
            Description("Multipart request Payload")
            Attribute("File", Bytes, "File")


I run the goa gen and the goa example commands to generate the boilerplate code. Apart from the cmd directory, the example code generates the images.go main file and a multipart.go file to declare the encoder and decoder logic, e.g.:

func ImagesUploadDecoderFunc(mr *multipart.Reader, p **images.UploadPayload) error {
    // Add multipart request decoder logic here
    return nil

I can use the mr.NextPart() and obtain a reference to the image file apparently, but I'm still not sure how should I map this to the Bytes field in the images.UploadPayload type (or maybe I should declare another type of field to handle Files??).

I can't find any example in the Goa documentation.