ASP.NET - Limit file upload available file types

I have added a file upload to my asp.net website. However, I want to limit the file types that user can select. For example, I only the user to select mp3 files. How can I add a filter to the file upload so that it displays only the mp3 files in the folder selected?

 <asp:FileUpload ID="FileUpload1" runat="server" />
 <asp:Button ID="btnAudUpload" Text="Upload" CssClass="btncssUpload" OnClick="btnAudUpload_Click" runat="server" />