golang detect content type of a file

detectcontenttype.go

file, err := os.Open(path)
if err != nil {
	return err
}
defer file.Close()

// Only the first 512 bytes are used to sniff the content type.
buffer := make([]byte, 512)
_, err = file.Read(buffer)
if err != nil {
	return err
}

// Reset the read pointer if necessary.
file.Seek(0, 0)

// Always returns a valid content-type and "application/octet-stream" if no others seemed to match.
contentType := http.DetectContentType(buffer)

// function still work, if size of buffer < 512
n, err = file.Read(buffer)
if err != nil && err != io.EOF {
return err
}
contentType := http.DetectContentType(buffer[:n])