Try this
<asp:fileupload id="fileupload1" runat="server" onchange="showButton()" xmlns:asp="#unknown" />
<asp:button id="btnUpload" runat="server" text="Upload" style="display:none" xmlns:asp="#unknown" />
<script type ="text/javascript" >
function showButton() {
document.getElementById("<%=btnUpload.ClientID%>").style.display = "";
}
</script>