VALIDATE FILE EXTENSION USING JAVASCRIPT

This Javascript can be used to allow only few file extension only.

Include Below Script
<script>
    function validateFileExtension(fld)
    {
        if(!/(\.zip|\.rar|\.doc|\.docx|\.ppt|\.xls|\.xlsx|\.pptx|\.jpg|\.jpeg|\.pdf)$/i.test(fld.value))         
       {
            alert("Invalid image file type.");
            fld.value='';
            fld.focus();
            return false;
        }
        return true;
    }
</script>

Call above function in HTML as given below:
<input type="file" name="uploadFile1" id="uploadFile1" onchange="return validateFileExtension(this)" />

Note:
Here as mention in Javascript only file with .zip, .rar, .doc, .docx, .ppt, .xls, .xlsx, .pptx, .jpg, .jpeg, .pdf  will be allowed. This can be changed as per requirement.