java 通过图片的Base64字符串判断文件格式

	/**
	 * base64字符串转写为文件
	 * 
	 * @param base64ImgData
	 * @param filePath
	 * @throws IOException
	 */
	public static void convertBase64DataToImage(String base64ImgData, String filePath) throws IOException {
		Files.write(Paths.get(filePath), Base64.getDecoder().decode(base64ImgData), StandardOpenOption.CREATE);
	}

	/**
	 * 将文件转写为base64字符串
	 * 
	 * @param filePath
	 * @return
	 * @throws IOException
	 */
	public static String convertImageToBase64Data(String filePath) throws IOException {
		byte[] b = Files.readAllBytes(Paths.get(filePath));
		return Base64.getEncoder().encodeToString(b);
	}

	/**
	 * 判断图片base64字符串的文件格式
	 * 
	 * @param base64ImgData
	 * @return
	 */
	public static String checkImageBase64Format(String base64ImgData) {
		byte[] b = Base64.getDecoder().decode(base64ImgData);
		String type = "";
		if (0x424D == ((b[0] & 0xff) << 8 | (b[1] & 0xff))) {
			type = "bmp";
		} else if (0x8950 == ((b[0] & 0xff) << 8 | (b[1] & 0xff))) {
			type = "png";
		} else if (0xFFD8 == ((b[0] & 0xff) << 8 | (b[1] & 0xff))) {
			type = "jpg";
		}
		return type;
	}