example code of uploading file on Goa v3

design.go

package design

import . "goa.design/goa/v3/dsl"

// TODO
// var _ = API("foo", func() { ....


var SourceFilePayload = Type("SourceFilePayload", func() {
	Attribute("file_name", String, "uploaded file name", func() {
		Example("foo.jpg")
	})
	Attribute("content", Bytes, "content of image")
	Attribute("format", String, "uploaded file format", func() {
		Example("image/jpeg")
	})
	Required(
		"file_name",
		"content",
		"format",
	)
})

var _ = Service("file", func() {
	HTTP(func() {
		Path("/file")
	})

	Method("upload", func() {
    Description("upload a file")
		Payload(SourceFilePayload)
		HTTP(func() {
			POST("/")
			MultipartRequest()
		})
	})
})

multipart.go

package foo

import (
	"bytes"
	"image"
	"io"
	"io/ioutil"
	"mime/multipart"
  
	_ "image/gif"
	_ "image/jpeg"
	_ "image/png"

	"foo/server/gen/file"
)

// FileUploadDecoderFunc implements the multipart decoder for service
// "file" endpoint "upload". The decoder must populate the argument p
// after encoding.
func FileUploadDecoderFunc(mr *multipart.Reader, p **file.SourceFilePayload) error {
	// Add multipart request decoder logic here
	var r file.SourceFilePayload
	for {
		part, err := mr.NextPart()
		if err == io.EOF {
			break
		}
		if err != nil {
			return err
		}
		if part.FileName() != "" {
			slurp, err := ioutil.ReadAll(part)
			if err != nil {
				return err
			}

			r.FileName = part.FileName()
			r.Content = slurp

			// guess mime type
			slurpReader := bytes.NewReader(slurp)
			_, ft, err := image.Decode(slurpReader)
			if err != nil {
				return err
			}
			r.Format = ft
			break
		}
	}
	*p = &r

	return nil
}