AjaxFileUpload's problem (Version 7.0429)

Use Version 7.0429 AJAX Control Toolkit 

AjaxFileUpload Control 

protected void AjaxFileUpload1_OnUploadComplete(object sender, AjaxFileUploadEventArgs file) 
{

AjaxFileUpload1.SaveAs(MapPath("~/App_Data/" + file.FileName));

} 

The Uploaded File : *.zip, *.txt ==> file can open 
The Uploaded File : *.doc, *.xls ..etc ==> file can't open, display "file is damaged",why?