a question for HtmlInputFile

I was junior programmar of .Net from China. I met a question about HtmlInputFile recently, I hope i can gain your help,Thanks!
  Describe the question as following:   First, I  did a Mastepage, I have added the properties of enctype="multipart/form-data" on <form id="form1" enctype="multipart/form-data" runat="server">   Then in the <asp:Content ID="Content1" ContentPlaceHolderID="ContentPlaceHolder1" Runat="Server">   I used a HTML control of inputFile, which is <input type="file" id="FileUpload2" runat="server" onchange="sf('ctl00_ContentPlaceHolder1_FileUpload2','ctl00_ContentPlaceHolder1_attName',false ...