Question

Does any one know how to read the header of a file in java using “magic numbers” or ascii values to get the name of the extension of a file

Answer 1

Maybe not the answer you wanted, but as you gave us very little information …

In unixoid systems (Linux, Mac, *BSD) you have the file command, that

tests each argument in an attempt to classify it. There are three sets of tests, performed in this order: filesystem tests, magic tests, and language tests. The first test that succeeds causes the file type to be printed.

E.g.

$ file linux-image-3.1.0-030100rc10-generic_3.1.0-030100rc10.201110200610_amd64.deb
linux-image-3.1.0-030100rc10-generic_3.1.0-030100rc10.201110200610_amd64.deb: Debian binary package (format 2.0)

Using Runtime.exec(…) you could invoke that program and parse its output.

Edit 1:

To determine if a given file is a PNG:

import java.io.*;

public class IsPng {

    public static void main(String ...filenames) throws Exception {
        if(filenames.length == 0) {
            System.err.println("Please supply filenames.");
            return;
        }

        for(String filename : filenames) {
            if(isPng(new File(filename))) {
                System.out.println(filename + " is a png.");
            } else {
                System.out.println(filename + " is _not_ a png.");
            }
        }
    }

    private static final int MAGIC[] = new int[] { 0x89, 0x50, 0x4e, 0x47, 0x0d, 0x0a, 0x1a, 0x0a };

    private static boolean isPng(File filename) throws Exception {
        FileInputStream ins = new FileInputStream(filename);
        try {
            for(int i = 0; i < MAGIC.length; ++i) {
                if(ins.read() != MAGIC[i]) {
                    return false;
                }
            }
            return true;
        } finally {
            ins.close();
        }
    }

}

Edit 2:

Sometimes URLConnection.getContentType() works, too, for local files:

new File(name).toURI().toURL().openConnection().getContentType()

But your comments sound like you have to implement the method by yourself, not using external programs (?).