func NewDecoder(r io.Reader, b string) *Decoder {
	d := new(Decoder)
	d.r = multipart.NewReader(r, b)
	return d
}
NewDecoder return new instance of Decoder
package mjpeg
import (
	"bytes"
	"errors"
	"fmt"
	"image"
	"image/jpeg"
	"io"
	"mime"
	"mime/multipart"
	"net/http"
	"net/textproto"
	"strings"
	"sync"
	"time"
)
type Decoder struct {
	r *multipart.Reader
	m sync.Mutex
}