<ajaxtoolkit:ajaxfileupload id="AjaxFileUpload1" xmlns:ajaxtoolkit="#unknown">
    ThrobberID="myThrobber"
    ContextKeys="fred"
    AllowedFileTypes="jpg,pdf"
    MaximumNumberOfFiles=10
    runat="server"/></ajaxtoolkit:ajaxfileupload>

将AllowedFileTypes设为pdf，试试吧

set AllowedFileTypes as pdf,try it

你可以通过客户端和服务器端验证来实现这一点

访问：

http://www.aspsnippets.com/Articles/ASPNet-FileUpload-File-Extension-Validation.aspx [ ^ ]

U can achieve this by both client and server side validations
Visit Here:
http://www.aspsnippets.com/Articles/ASPNet-FileUpload-File-Extension-Validation.aspx[^]