<asp:Button runat="server" ID="btnSubmit" Text="提交" onsubmit="GetFileList();" OnClick="btnSubmit_Click" 
                    class="butt" />

function GetFileList() {
            alert('submit');
            var fileList = document.getElementsByTagName("input");
            if (fileList != null && fileList.length > 0) {
                var count = fileList.length;
                var countIn = fileList.length;
                for (var i = 0; i < count; i++) {
                    if (fileList[i].type.toString() == "file") {
                        var fuMain = fileList[i];
                        if (fuMain != null) {
                            if (fuMain.value == "") {
                                countIn--;
                            }
                        }
                    }
                }
                if (countIn == count) {
                    for (var i = 0; i < count; i++) {
                        if (fileList[i].type.toString() == "file") {
                            var fuMain = fileList[i];
                            if (fuMain != null) {
                                document.getElementById("hfFile").value += fuMain.value + "|";
                            }
                        }
                    }
                } else {
                    alert('请上传文件!');
                    return;
                 }
            }
        }

protected void btnSubmit_Click(object sender, EventArgs e)
    {
内容省。。
}