Server control INPUT type="file" in a datalist

[fchiausa]

Hello,
I'm having a hard time trying to manage an input type file in a
datalist here is my aspx code :
<INPUT type="file" id="InputImageFile2" runat="server"><br>
<input type="submit" value="upload" onserverclick="Upload_Click"
runat="server">

here is my code behind :
protected System.Web.UI.HtmlControls.HtmlInputFile InputImageFile2;//I
have to declare it my self cause Visual didn't do it !
public void Upload_Click(object sender, System.EventArgs e)
{
string strFileName;
string strFolder;
strFolder = Server.MapPath("./") ;

// Retrieve the name of the file that is posted.
strFileName = InputImageFile2.PostedFile.FileName;
}
My problem is we the server tries to execute the line code :
strFileName = InputImageFile2.PostedFile.FileName; I get the error :
System.NullReferenceException: La référence d'objet n'est pas définie
à une instance d'un objet. witch means that the reference to the
object is not define (or something like this in English)

How can I solve the problem ?
PS If it put my code outside of the datalist it works fine.