I give user the option of uploading a file like this

<form action=# onsubmit=return Checkfiles(this);>

    <center><input type=file id=filename> 

    <input type=submit value=Go!></center>

    </form>

When user uploads the file, I validate the file using the following javascript function

<script type=text/javascript>

        function Checkfiles()

        {

        var fup = document.getElementById('filename');

        var fileName = fup.value;

        var ext = fileName.substring(fileName.lastIndexOf('.') + 1);

        if(ext == pdf )

        {

        return true;

        } 

        else

        {

        alert(Upload pdf files only);

        fup.focus();

        return false;

        }

        }

    </script>

Evertything is working fine with it.

But I want to validate file by some other means and not just by its extension

I will give reason for doing this. I renamed an image file to image.pdf and now its in pdf format but couldnt be opened.

So I want to validate a pdf file in exact way. Is there any other way to check other than by its extension?

Edit

I want to do validation at server end using jsp page. So what should I do for that?

Thanks in ADVANCE :)