Multipart file field is unreadable

I am trying to upload photos to Twitter. I created a multipart writer and creating a file field using that named media but when I send my request to Twitter it keeps responding missing media field.
Am I missing something?
Here is my code

	f, err := os.Open("/Users/nikos/Desktop/test.png")
errored:
if nil != err {
fmt.Println(err)
return
}
var img = new(bytes.Buffer)
enc := base64.NewEncoder(base64.StdEncoding, img)
_, err = io.Copy(enc, f)
if nil != err {
goto errored
}
body := new(bytes.Buffer)//Multipart body
writer := multipart.NewWriter(body)
cl, err := twitter.OauthClient.MakeHttpClient(&oauth.AccessToken{
Token:  "xxx",
Secret: "yyy",
})
err = writer.WriteField("media_data", img.String())//base64 version of the image (i tried both binary and base64 versions neither will work)
if nil != err {
goto errored
}
part, err := writer.CreateFormFile("media", "test.png")//actual binary file multiparted and it is named media.
if nil != err {
goto errored
}
_, err = io.Copy(part, f)
if nil != err {
goto errored
}
req, err := http.NewRequest("POST",
"https://upload.twitter.com/1.1/media/upload.json",
body)
if nil != err {
goto errored
}
res, err := cl.Do(req)
if nil != err {
goto errored
}
//and twitter responds that there is no field attached named media
_, err = io.Copy(os.Stdout, res.Body)
fmt.Println(res)
if nil != err {
goto errored
} 

Updates: Just referred Twitter API Upload parameter. As per your code snippet you're using both fields media and media_data. You have to use only one -

• Upload using base64 -> field name is media_data
• Upload using raw -> field name is media

And, you have to add Content-Type header.

req, err := http.NewRequest("POST",
"https://upload.twitter.com/1.1/media/upload.json",
body)
req.Header.Set("Content-Type", writer.FormDataContentType())
if err := writer.Close(); err != nil {
log.Println(err)
}
// Now fire the http request

PS: While composing an answer, in 30 secs gap, @cerise-limón added comment, also close the multipart writer as mentioned by @cerise-limón.

Asked in the comment:

> Twitter accepts application/octet-stream, you may not need below approach.

Adding multi-part with user supplied Content-Type instead of application/octet-stream. Basically you have to do same implementation as convenience wrapper with your content-type.

writer := multipart.NewWriter(body)
h := make(textproto.MIMEHeader)
h.Set("Content-Disposition", fmt.Sprintf(`form-data; name="%s"; filename="%s"`,
escapeQuotes(fieldname), escapeQuotes(filename)))
h.Set("Content-Type", "image/png")
part, err := writer.CreatePart(h)
// use part same as before

Definition of escapeQuotes from multiple-part package.

var quoteEscaper = strings.NewReplacer("\\", "\\\\", `"`, "\\\"")
func escapeQuotes(s string) string {
return quoteEscaper.Replace(s)
}