What is the best approach to define a specific file type in Java?

I am trying to use the following approach to identify specific file types using Java. I need to implement things like this in my web application.

package filetype;

import java.io.File;
import java.net.URLConnection;

final public class FileType
{
    public static void main(String[] args)
    {
        File temp=new File("G:/mountain.jpg");
        System.out.println(URLConnection.guessContentTypeFromName(temp.getAbsolutePath()));

        temp=new File("G:/myFile.txt");
        System.out.println(URLConnection.guessContentTypeFromName(temp.getAbsolutePath()));

        temp=new File("G:/zipByJava.zip");
        System.out.println(URLConnection.guessContentTypeFromName(temp.getAbsolutePath()));

        temp=new File("G:/MLM/Login.aspx");
        System.out.println(URLConnection.guessContentTypeFromName(temp.getAbsolutePath()));

        temp=new File("G:/power_point.pptx");
        System.out.println(URLConnection.guessContentTypeFromName(temp.getAbsolutePath()));

        temp=new File("G:/excel_sheet.xlsx");
        System.out.println(URLConnection.guessContentTypeFromName(temp.getAbsolutePath()));

        temp=new File("G:/word_document.docx");
        System.out.println(URLConnection.guessContentTypeFromName(temp.getAbsolutePath()));
    }
}

      

        
        
        
      

    

It displays the following output to the console.

image/jpeg
text/plain
application/zip
null
null
null
null

      

        
        
        
      

    

In the last four cases, it displays null

and cannot recognize the file type of this file. What is the best approach for detecting a specific file type in Java?