ajaxToolkit:AjaxFileUpload capture file name and filetype

I am using AjaxFileUpload to upload the files.

1. I need to allow users to only upload text file. I used  AllowedFileTypes="txt" and it does not work.

<ajaxToolkit:AjaxFileUploadID="AjaxFileUpload1" runat="server"OnUploadComplete="AjaxFileUpload1_UploadComplete"AllowedFileTypes="txt"ClientIDMode="AutoID"OnClientUploadError="uploadError"MaximumNumberOfFiles="10"/>    

2. I need to allow users to upload only files that  starts with SAGdesc.txt, SAGhelp.txt. How do I check the file names before uploading. If the file name is other than display a message to upload only files that start with SAGdesc and SAGhelp.

Thanks.