<telerik:RadTabStrip ID="RtsBug" runat="server" MultiPageID="RmpBug" OnClientTabSelected="onTabSelected">
    <KeyboardNavigationSettings CommandKey="Alt" FocusKey="M" />
    <Tabs>
        <telerik:RadTab runat="server" PageViewID="RadPageView1" Text="Textbox" />
        <telerik:RadTab runat="server" PageViewID="RadPageView2" Text="Upload" />
    </Tabs>
</telerik:RadTabStrip>

<telerik:RadMultiPage ID="RmpBug" runat="server">
    <telerik:RadPageView ID="RadPageView1" runat="server">
        <telerik:RadTextBox runat="server" />
        <telerik:RadAsyncUpload ID="RadAsyncUpload2" runat="server" EnableAriaSupport="true"></telerik:RadAsyncUpload>
    </telerik:RadPageView>
    <telerik:RadPageView ID="RadPageView2" runat="server">
        <telerik:RadAsyncUpload ID="RadAsyncUpload1" runat="server" EnableAriaSupport="true"></telerik:RadAsyncUpload>
    </telerik:RadPageView>
</telerik:RadMultiPage>

function onTabSelected(sender, args) {
    var text = args.get_tab().get_text();

    if (text === "Upload") {
        setTimeout(function () {
            var uploadElement = $find("<%= RadAsyncUpload1.ClientID %>");
            uploadElement._fileInput.focus();
        }, 100);
    }
}