See
AsyncFileUpload is an ASP.NET AJAX Control that allows you asynchronously upload files to server. The file uploading results can be checked both in the server and client sides.
<ajaxToolkit:AsyncFileUpload OnClientUploadError="uploadError"
OnClientUploadComplete="uploadComplete" runat="server"
ID="AsyncFileUpload1" Width="400px" UploaderStyle="Modern"
UploadingBackColor="#CCFFFF" ThrobberID="myThrobber"
/>
Get some good links
http://www.asp.net/ajaxLibrary/AjaxControlToolkitSampleSite/AsyncFileUpload/AsyncFileUpload.aspx
http://csharpdotnetfreak.blogspot.com/2012/08/ajaxfileupload-asyncfileupload-in-modalpopup.html