a server converting raw RGBA to bmp

main.go

package main
import(
  "golang.org/x/image/bmp"
  "image"
  "io/ioutil"
  "fmt"
  "strconv"
  "net/http"
  "log"
  //"mime/multipart"
)
func main(){
  http.HandleFunc("/up",func(w http.ResponseWriter, r*http.Request){
    if r.Method == "GET"{
      w.Header().Add("content-type","text/html")
      fmt.Fprint(w, `<form enctype="multipart/form-data" action="/up" method="post">
      <input type="file" name="uploadfile"/>
      <input type="text" name="width"/>
      <input type="submit" value="submit"/>
      </form>
      `)
      return
    }
    err := r.ParseMultipartForm(25 * 1024 * 1024)//25M
    if err != nil{
      http.Error(w, err.Error(), 500);
      return
    }
    defer r.Body.Close()

    file, _, err := r.FormFile("uploadfile")
    if err != nil {
      http.Error(w, err.Error(), 500);
      return
    }
    defer file.Close()

    widthStr := r.FormValue("width")
    if widthStr == ""{
      widthStr = "1080"
    }
    width64,err := strconv.ParseInt(widthStr, 10, 32)
    if err!=nil{
      http.Error(w, err.Error(), 500);
      return
    }
    width := int(width64)
    raw, err := ioutil.ReadAll(file)
    if err != nil{
      http.Error(w, err.Error(), 500);
      return
    }
    rect := image.Rect(0,0,int(width),len(raw)/4/int(width))
    rgba := image.RGBA{
      raw,
      4*int(width),
      rect,
    }
    img := rgba.SubImage(rect)
    w.Header().Add("content-type", "image/bmp")
    err = bmp.Encode(w, img);
    if err!=nil{
      http.Error(w, err.Error(), 500);
    }
  })

  log.Fatal(http.ListenAndServe(":3333", nil))
}