đúng

<form runat="server" id="from" enctype="multipart/form-data" >  để đảm bảo không sai xót
HTML File
--------------------------------
<script language="javascript">
var url = "ajax/ajaxFile.aspx";
var binary;
var filename;
var mytext;

function upload() {
   filename = document.getElementById('myfile').value;
   mytext = document.getElementById('mytext').value;
   document.getElementById('ajaxbutton').disabled = true;

   // request local file read permission
   try {
       netscape.security.PrivilegeManager.enablePrivilege("UniversalXPConnect");
   } catch (e) {
       alert("Permission to read file was denied.");
   }
   
   // open the local file
   var file = Components.classes["@mozilla.org/file/local;1"]
       .createInstance(Components.interfaces.nsILocalFile);
   file.initWithPath( filename );        
   stream = Components.classes["@mozilla.org/network/file-input-stream;1"]
       .createInstance(Components.interfaces.nsIFileInputStream);
   stream.init(file,    0x01, 00004, null);
   var bstream =  Components.classes["@mozilla.org/network/buffered-input-stream;1"]
       .getService();
   bstream.QueryInterface(Components.interfaces.nsIBufferedInputStream);
   bstream.init(stream, 1000);
   bstream.QueryInterface(Components.interfaces.nsIInputStream);
   binary = Components.classes["@mozilla.org/binaryinputstream;1"]
       .createInstance(Components.interfaces.nsIBinaryInputStream);
   binary.setInputStream (stream);

   // start AJAX file upload in 1 second
   window.setTimeout("ajax_upload()", 1000);
}

function ajax_upload() {
   // request more permissions
   try {
       netscape.security.PrivilegeManager.enablePrivilege("UniversalXPConnect");
   } catch (e) {
       alert("Permission to read file was denied.");
   }

   http_request = false;
   http_request = new XMLHttpRequest();
   if (!http_request) {
       alert('Cannot create XMLHTTP instance');
       return false;
   }

   // prepare the MIME POST data
   var boundaryString = 'capitano';
   var boundary = '--' + boundaryString;
   var requestbody = boundary + '\n'
   + 'Content-Disposition: form-data; name="mytext"' + '\n'
   + '\n'
   + mytext + '\n'
   + '\n'
   + boundary + '\n'
   + 'Content-Disposition: form-data; name="myfile"; filename="'
       + filename + '"' + '\n'
   + 'Content-Type: application/octet-stream' + '\n'
   + '\n'
   + escape(binary.readBytes(binary.available()))
   + '\n'
   + boundary;

   document.getElementById('sizespan').innerHTML =
       "requestbody.length=" + requestbody.length;
   
   // do the AJAX request
   http_request.onreadystatechange = requestdone;
   http_request.open('POST', url, true);
   http_request.setRequestHeader("Content-type", "multipart/form-data; \
       boundary=\"" + boundaryString + "\"");
   http_request.setRequestHeader("Connection", "close");
   http_request.setRequestHeader("Content-length", requestbody.length);
   http_request.send(requestbody);

}

function requestdone() {
   if (http_request.readyState == 4) {
       if (http_request.status == 200) {
           result = http_request.responseText;
           document.getElementById('myspan').innerHTML = result;            
       } else {
           alert('There was a problem with the request.');
       }
       document.getElementById('ajaxbutton').disabled = false;
   }
}

</script>

<form>
Text: <input type="text" id="mytext" name="mytext" size="40">
<br>
File: <input type="file" id="myfile" name="datafile" size="40"><br>
<input type="button" id="ajaxbutton" value="AJAX IT" onclick="upload();">
</form>

<div id="sizespan"></div>
<hr>
<div id="myspan"></div>
--------------------------------------------------------------
asp.net File
protected void Page_Load(object sender, EventArgs e)
   {
       if (Request.Files.Count > 0)
       {
           try
           {
               for (int i = 0; i < Request.Files.Count; i++)
               {

                   int values = Request.Files.FileName.LastIndexOf('\\');
                   string filename = Request.Files.FileName.Substring(values + 1);
                   String strSavePath = Server.MapPath(ConfigurationManager.AppSettings["NewsImage"]) +"\\"+ filename;
                   Response.Write(strSavePath);
                   Request.Files.SaveAs(strSavePath);
               }
           }
           catch(Exception objEx)
           {
               Response.Write(objEx.Message);
           }
       }
       else
       {
           Response.Write(" khong co file");
       }
   }