使用RegularExpressionValidator验证ASP.NET FileUpload控件中的文件类型时出现问题

code:

<span>Upload Adobe Acrobat file<img src="../../Images/UI/pdf.jpg" style="height: 25;
    width: 20" height="25" width="20" /></span>
<asp:FileUpload ID="uplPdf" runat="server" />
<asp:RegularExpressionValidator ID="valPdf" runat="server" ErrorMessage="Only PDF files are allowed!"
    ValidationExpression="^(([a-zA-Z]:)|(\{2}\w+)$?)(\(\w[\w].*))(.pdf)$" Display="Dynamic"
    ControlToValidate="uplPdf" ValidationGroup="upload" />
<br />
<asp:Button ID="btnUpload" runat="server" Text="Upload files" OnClick="btnUpload_Click"
    ValidationGroup="upload" />
<span>Upload Adobe Acrobat file<img src