for f in request.files.getlist("files[]"):
        if filename_allowed(f.filename):
            txt = f.read().decode("utf-8")
            report += f"{f.filename} : {txt} \n"
        else:
            report += f"{f.filename} nicht gültig. \n"
        
        # with open("Output.txt", "w") as text_file:
        #     text_file.write(report)
        report = "<h1> master </h1>"

    <form enctype='multipart/form-data' method='POST' action='/upload_files'> 
        <input type='file' name='files[]' accept=".txt,.TXT" multiple/>
        <button type='submit'>Submit</button>
    </form>