code: function getOrientation(buffer){
      var view = new DataView(buffer);
      if (view.getUint16(0, false) != 0xFFD8) return -2;
      var length = view.byteLength, offset = 2;
      while (offset < length) {
          var marker = view.getUint16(offset, false);
          offset += 2;
          if (marker == 0xFFE1) {
              if (view.getUint32(offset += 2, false) != 0x45786966) return -1;
              var little = view.getUint16(offset += 6, false) == 0x4949;
              offset += view.getUint32(offset + 4, little);
              var tags = view.getUint16(offset, little);
              offset += 2;
              for (var i = 0; i < tags; i++)
                  if (view.getUint16(offset + (i * 12), little) == 0x0112)
                      return view.getUint16(offset + (i * 12) + 8, little);
          }
          else if ((marker & 0xFF00) != 0xFF00) break;
          else offset += view.getUint16(offset, false);
      }
      return -1;
  }
docs: 获取图片的orientation
  ref to http://stackoverflow.com/questions/7584794/accessing-jpeg-exif-rotation-data-in-javascript-on-the-client-side