golang image.DecodeConfig image: unknown format

package main
import (
    "fmt"
    "image" 
    "io/ioutil"
    "os"
    "path/filepath"
)
const dir_to_scan string = "/home/da/to_merge"
func main() {
    files, _ := ioutil.ReadDir(dir_to_scan)
    for _, imgFile := range files {
        if reader, err := os.Open(filepath.Join(dir_to_scan, imgFile.Name())); err == nil {
            defer reader.Close()
            im, _, err := image.DecodeConfig(reader)
            if err != nil {
                fmt.Fprintf(os.Stderr, "%s: %v
", imgFile.Name(), err)
                continue
            }
            fmt.Printf("%s %d %d
", imgFile.Name(), im.Width, im.Height)
        } else {
            fmt.Println("Impossible to open the file:", err)
        }
    }
}

import (
    _"image/gif"
    _ "image/jpeg"
    _ "image/png"
)