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Button doesn't fire event when filename not exist on firefox
<%@ Page Language="C#" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<script runat="server">

    protected void UploadButton_Click(object sender, EventArgs e)
    {
        if (FileUpload1.HasFile)
        {
            UploadStatusLabel.Text = "Your file wasn't saved. It's a test.";
        }
        else
        {
            // Notify the user that a file was not uploaded.
            UploadStatusLabel.Text = "You did not specify a file to upload.";
        }
    }
</script>
<html xmlns="http://www.w3.org/1999/xhtml">
<head id="Head1" runat="server">
    <title>FileUpload Example</title>
</head>
<body>
    <form id="form1" runat="server">
    <div>
        <h4>
            Select a file to upload:</h4>
        <asp:FileUpload ID="FileUpload1" runat="server"></asp:FileUpload>
        <br />
        <br />
        <asp:Button ID="UploadButton" Text="Upload file" OnClick="UploadButton_Click" runat="server">
        </asp:Button>
        <hr />
        <asp:Label ID="UploadStatusLabel" runat="server">
        </asp:Label>
    </div>
    </form>
</body>
</html>