Golang文件上传：如果文件太大，则关闭连接

I want to allow uploading files. Go is being used server-side to handle requests. I would like to send a response "File too large" whenever the file they're trying to upload is too large. I would like to do so, before the entire file is uploaded (bandwidth).

I am using the following snippet, but it's only sending a response after the client is done uploading. It saves a 5 kB file.

const MaxFileSize = 5 * 1024
// This feels like a bad hack...
if r.ContentLength > MaxFileSize {
    if flusher, ok := w.(http.Flusher); ok {
        response := []byte("Request too large")
        w.Header().Set("Connection", "close")
        w.Header().Set("Content-Length", fmt.Sprintf("%d", len(response)))
        w.WriteHeader(http.StatusExpectationFailed)
        w.Write(response)
        flusher.Flush()
    }
    conn, _, _ :=  w.(http.Hijacker).Hijack()
    conn.Close()
    return
}

r.Body = http.MaxBytesReader(w, r.Body, MaxFileSize)

err := r.ParseMultipartForm(1024)
if err != nil {
    w.Write([]byte("File too large"));
    return
}

file, header, err := r.FormFile("file")
if err != nil {
    panic(err)
}

dst, err := os.Create("upload/" + header.Filename)
defer dst.Close()
if err != nil { 
    panic(err)
}

written, err := io.Copy(dst, io.LimitReader(file, MaxFileSize))
if err != nil {
    panic(err)
}

if written == MaxFileSize {
    w.Write([]byte("File too large"))
    return
}
w.Write([]byte("Success..."))