def clean_file_name(filename, unique=True, replace="_", force_nt=False):
    """
    Return a filename version, which has no characters in it which are forbidden.
    On Windows these are for example <, /, ?, ...

    The intention of this function is to allow distribution of files to different OSes.

    :param filename: string to clean
    :param unique: check if the filename is already taken and append an integer to be unique (default: True)
    :param replace: replacement character. (default: '_')
    :param force_nt: Force shortening of paths like on NT systems (default: False)
    :return: clean string
    """

    if re.match(r'[<>:"/\\|?* .\x00-\x1f]', replace):
        raise ValueError("replacement character is not allowed!")

    path, fname = os.path.split(filename)
    # For Windows see: https://msdn.microsoft.com/en-us/library/windows/desktop/aa365247(v=vs.85).aspx
    # Other operating systems seems to be more tolerant...

    # Not allowed filenames, attach replace character if necessary
    if re.match(r'(CON|PRN|AUX|NUL|COM[1-9]|LPT[1-9])', fname):
        fname += replace

    # reserved characters
    fname = re.sub(r'[<>:"/\\|?*\x00-\x1f]', replace, fname)
    # Do not end with dot or space
    fname = re.sub(r'[ .]$', replace, fname)

    if force_nt or os.name == 'nt':
        PATH_MAX_LENGTH = 230  # give extra space for other stuff...
        # Check filename length limit, usually a problem on older Windows versions
        if len(fname) > PATH_MAX_LENGTH:
            if "." in fname:
                f, ext = fname.rsplit(".", 1)
                fname = "{}.{}".format(f[:PATH_MAX_LENGTH-(len(ext)+1)], ext)
            else:
                fname = fname[:PATH_MAX_LENGTH]

        # Special behaviour... On Windows, there is also a problem with the maximum path length in explorer.exe
        # maximum length is limited to 260 chars, so use 250 to have room for other stuff
        if len(os.path.abspath(os.path.join(path, fname))) > 250:
            fname = fname[:250 - (len(os.path.abspath(path)) + 1)]

    if unique:
        counter = 0
        origname = fname
        while os.path.isfile(os.path.join(path, fname)):
            if "." in fname:
                # assume extension
                f, ext = origname.rsplit(".", 1)
                fname = "{}_{}.{}".format(f, counter, ext)
            else:
                fname = "{}_{}".format(origname, counter)
            counter += 1

    return os.path.join(path, fname)