Replace file extension
Replace file extension box is used to hide some file extension behind .htm or .html extensions. You can use only 3 or 4 chars file extensions here. All files with .htm or .html extensions within the transformation scope will be rewritten to the entered file extension. But your real .htm files will remain working fine, because LinkFreeze can determine physical file request and pass it through. But LinkFreeze technology requires to preserve link length so 3 character file extensions can only be rewritten as another 3 character file extension. This means that .aspx file extension will be replaced by .html and .asp file extension can only be replaced by .htm extension.