List files from a directory by approach

I have a screen where listo files of a certain folder. I have a search bar that uses the same method to find the files, but I can not list by approximation. Example:

I have this file Papel_timbrado.docx .

If I search "Paper" I find the file, however if I search "Letterhead" my search returns blank. is there any way to find the files by approximation?

I'm using the System.IO class to find the files ( GetFiles )

    public void PopulaLista(string arquivo, string extensao)
    {

        string resposta = "<table class=\"table table-striped table-bordered table_qr\" id=\"sample_1\" runat=\"server\">";
        DirectoryInfo diretorio = new DirectoryInfo(objUtils.Diretorio("upload/pastamateriais/"));

        if (string.IsNullOrWhiteSpace(arquivo) == true) { arquivo = "*"; }
        FileInfo[] Arquivos = diretorio.GetFiles(arquivo + extensao);

        resposta += " <thead>";
        resposta += "     <tr>";

        resposta += "         <th>Nome do Arquivo</th>";
        resposta += "         <th>Tipo</th>";
        resposta += "         <th>Tamanho</th>";
        resposta += "         <th style=\"width:70px;\">Ações</th>";
        resposta += "     </tr>";
        resposta += " </thead>";

        resposta += " <tbody>";
        TextInfo myTI = new CultureInfo("en-US", false).TextInfo;
        foreach (FileInfo fileinfos in Arquivos)
        {
            if (fileinfos.Name != "Thumbs.db")
            {

                resposta += " <tr>";
                resposta += "     <td>" + myTI.ToTitleCase(fileinfos.Name.Replace(fileinfos.Extension, "").Replace("_", " ").Replace("-"," ")) + "</td>";
                resposta += "     <td><img alt='" + fileinfos.Extension + "' src='images/icon-" + fileinfos.Extension.Replace(".", "") + ".png' /></td>";
                resposta += "     <td>" + Convert.ToDecimal(fileinfos.Length / 1024) + " KB</td>";
                resposta += "     <td><a href=upload/pastamateriais/" + fileinfos.Name + " class=\"img_edit\" download><img src=\"images/download.png\" /></a><a id='" + fileinfos.Name + "' href=\"javascript:void(0)\" onclick='apagar(this.id);' class=\"img_del\"><img src=\"images/lixo.png\" /></a></td>";
                resposta += " </tr>";
            }

        }

        resposta += " </tbody>";

        resposta += "</table>";

        if (Request["acao"] == "Pesquisar")
        {
            Response.Write(resposta);
        }
        else
        {
            divLista.InnerHtml = resposta;
        }

    }