java中判断文件是否是图片，不是用扩展名
来源于网络资源，太多出处了，不知道哪个是原创，- -!

   /**     * 检查传入file的format名称     * @param f 传入的文件     * @return     */    public static String getFormatInFile(File f) {        return getFormatName(f);    }    /**     * 对文件进行format检索     * .jpg .jpeg .jpe .jfif ===> JPEG     * .png ===> png     * .gif ===> gif     * .     * @param o     * @return     */    // Returns the format name of the image in the object 'o'.    // Returns null if the format is not known.    private static String getFormatName(Object o) {        try {            // Create an image input stream on the image            ImageInputStream iis = ImageIO.createImageInputStream(o);            // Find all image readers that recognize the image format            Iterator<ImageReader> iter = ImageIO.getImageReaders(iis);            if (!iter.hasNext()) {                // No readers found                return null;            }            // Use the first reader            ImageReader reader = (ImageReader) iter.next();            // Close stream            iis.close();            // Return the format name            return reader.getFormatName();        } catch (IOException e) {            //        }        // The image could not be read        return null;    }