Add this javascript function.
function clickfp() {
var objFp = document.getElementById('<%= fpUpload.ClientID %>');
objFp.click();
}
Call javascript function on hyperlink onclick.
<input type="file" id="fpUpload" runat="server" name="filediag" style="display:none" />
<asp:HyperLink ID="hlClick" runat="server" Text="Upload" onclick="clickfp();"></asp:HyperLink>