golang server upload file response net::ERR_EMPTY_RESPONSE - file-upload

Question

I am uploading a small audio file to server using DART + golang. Everything kinda works fine, until I POST and go doesn't return anything. I would like to return filename so I can change the label text on the input.
1) GOLANG:
import (
"encoding/json"
"io/ioutil"
"log"
"net/http"
"time"
"fmt"
"os"
"io"
)
http.HandleFunc("/upload", webUploadHandler)
[...]
func webUploadHandler(w http.ResponseWriter, r *http.Request) {
file, header, err := r.FormFile("file") // the FormFile function takes in the POST input id file
defer file.Close()
if err != nil {
fmt.Fprintln(w, err)
return
}
out, err := os.Create("/tmp/uploadedfile")
if err != nil {
fmt.Fprintf(w, "Unable to create the file for writing. Check your write access privilege")
return
}
defer out.Close()
// write the content from POST to the file
_, err = io.Copy(out, file)
if err != nil {
fmt.Fprintln(w, err)
}
fmt.Fprintf(w,"File uploaded successfully : ")
fmt.Fprintf(w, header.Filename)
}
2) DART response, alert
window.alert("upload complete");
works
3) ERROR in Chromium Console:
POST http://localhost:9999/upload net::ERR_EMPTY_RESPONSE
I'm quite new to GOLANG so any help will me much appreciated.

Answer 1

First error in the code above:
defer file.Close()
was before checking
if err != nil
-- UPDATE
and missing part 2, in DART:
req.setRequestHeader("Content-type","multipart/form-data");