function change()
  { 
    var f=document.getElementById("pphoto").value;
    var len1=f.length;
    var exte=f.substring(len1-4,len1);
    alert(exte);
 
    if(exte==".jpg")
    document.getElementById("nextcontrol").focus();
    else
     {
       alert("File extension must be .jpg"); 
       document.getElementById("pphoto").focus();
       document.getElementById("pphoto").value="";
    
     }
   }

<asp:FileUpload ID="pphoto" runat="server"  OnChange="change();" Width="367px" />