import java.io.File;
import java.io.IOException;
import java.util.Iterator;

import javax.imageio.ImageIO;
import javax.imageio.ImageReader;
import javax.imageio.stream.ImageInputStream;

public class DetermineFormatOfAnImage {

    public static void main(String[] args) throws IOException {

        // get image format in a file
        File file = new File("newimage.jpg");

        // create an image input stream from the specified file
        ImageInputStream iis = ImageIO.createImageInputStream(file);

        // get all currently registered readers that recognize the image format
        Iterator<ImageReader> iter = ImageIO.getImageReaders(iis);

        if (!iter.hasNext()) {
            throw new RuntimeException("No readers found!");
        }

        // get the first reader
        ImageReader reader = iter.next();

        System.out.println("Format: " + reader.getFormatName());

        // close stream
        iis.close();

    }

}