Hi Ken,

Actually on the  page there could be more than 1 Ajaxuploader at a time , no of Ajaxuploader are decide at runtime,

so

var btnUpload = document.getElementById("<%= btnUpload.ClientID%>"); 
it will not get the actual btnupload   client id

example belows shows:-

I am not sure above image is visible to you or not , if it is not ican email. 

 Thanks

Rajiv