modified logic for checking JPEG format.
Created by: 10km
modified logic judgment for checking JPEG format. JFIF and Exif is most popular file format of JPEG encoded image,but not all.for MJPEG format that used by camera,the flag is 'AVI1' OR not any flag ,starting FFC0 after FFD8. so the old logic is not work for other JPEG format. see my blog for detail : https://blog.csdn.net/10km/article/details/82155967