Detect uploaded file type

main.go

func uploadImage(w http.ResponseWriter, r *http.Request) {
    // Create buffer to read magic number of uploaded file
    magicNumber := make([]byte, 3)

    // Read first 3 bytes of uploaded file
    _, err := r.Body.Read(magicNumber)
    // handle err

    // Check magic number against png, jpg, gif
    // https://en.wikipedia.org/wiki/List_of_file_signatures
    ext := ""
    switch {
    case bytes.Compare(magicNumber, []byte{0xFF, 0xD8, 0xFF}) == 0: // ÿØÿ (JPG)
        log.Println("file is a jpg")
        ext = ".jpg"

    case bytes.Compare(magicNumber, []byte{0x89, 0x50, 0x4E}) == 0: // .PNG (PNG)
        log.Println("file is a png")
        ext = ".png"

    case bytes.Compare(magicNumber, []byte{0x47, 0x49, 0x46}) == 0: // GIF (GIF)
        log.Println("file is a gif")
        ext = ".gif"

    default:
        log.Println("file is neither")
        http.Error(w, "invalid file", http.StatusBadRequest)
        return
    }

    // Stitch the bytes back together
    fileReader := io.MultiReader(bytes.NewReader(magicNumber), r.Body)

    // Create new file for storage
    out, err := os.Create("thefile" + ext)
    // handle err

    // Save file to disk
    io.Copy(out, fileReader)

    // Or in your case, respond back to client
    io.Copy(w, fileReader)    
}