Golang : How to verify uploaded file is image or allowed file types

Problem :

How do I verify if an uploaded file to my server is ... let say I only allow images(JPG, PNG, GIF)... is indeed image file.

Solution :

Use DetectContentType function to detect the uploaded file. Only proceed further if the uploaded file is image or allowed file types.

If not, abort and prompt error message.

The code will assume that the target file is uploaded from client. To see how to upload file in Golang, please visit https://www.socketloop.com/tutorials/golang-upload-file

Codes :

 package main

 import (
 "fmt"
 "net/http"
 "os"
 "runtime"
 )

 func main() {

 // maximize CPU usage for maximum performance
 runtime.GOMAXPROCS(runtime.NumCPU())

 // open the uploaded file
 file, err := os.Open("./img.png")

 if err != nil {
 fmt.Println(err)
 os.Exit(1)
 }

 buff := make([]byte, 512) // why 512 bytes ? see http://golang.org/pkg/net/http/#DetectContentType
 _, err = file.Read(buff)

 if err != nil {
 fmt.Println(err)
 os.Exit(1)
 }

 filetype := http.DetectContentType(buff)

 fmt.Println(filetype)

 switch filetype {
 case "image/jpeg", "image/jpg":
 fmt.Println(filetype)

 case "image/gif":
 fmt.Println(filetype)

 case "image/png":
 fmt.Println(filetype)

 case "application/pdf": // not image, but application !
 fmt.Println(filetype)
 default:
 fmt.Println("unknown file type uploaded")
 }

 }