Help, Why can't upload file from normal html form ?

[new to add-in]

In asp.net/c#, it's easy to implement Upload file to web server, just use
Request.Files or Request.InputSt ream & runat=server form and type=file input
controls. But now, it's a problem for me how to invoke a .aspx code to upload
file from a outer html file or c/s application.

I want to upload file by my small vb6.0 program using MS Winsock Control,
just invoke web server side file "UploadFile.asp x". So firstly, I used the
following "testUploadFile .htm" to test "UploadFile.asp x" :

content of testUploadFile. htm :

<form id="Form1" method="post" action="UploadF ile.aspx" enctype =
"multipart/form-data" >
<INPUT id=uploadfile1 type=file size=49 >
<input type=submit value="submit">
</form>
In UploadFile.aspx , Not any "runat=serv er" control, just only following
simple codes in UploadFile.aspx .cs :

private void Page_Load(objec t sender, System.EventArg s e)
{
Response.Write( "uploaded file(s) count: " +
System.Web.Http Context.Current .Request.Files. Count.ToString( ));
}

But it always prompted "uploaded file(s) count: 0", just can't get any file.
Thanks for any suggestion!
David W