def format_name_for_ext(metadata, ext, cm_default_formats=None, explicit_default=True):
    """Return the format name for that extension"""
    # Is the format information available in the text representation?
    text_repr = metadata.get('jupytext', {}).get('text_representation', {})
    if text_repr.get('extension', '').endswith(ext) and text_repr.get('format_name'):
        return text_repr.get('format_name')

    # Format from jupytext.formats
    formats = metadata.get('jupytext', {}).get('formats', '') or cm_default_formats
    formats = long_form_multiple_formats(formats)
    for fmt in formats:
        if fmt['extension'] == ext:
            if (not explicit_default) or fmt.get('format_name'):
                return fmt.get('format_name')

    if (not explicit_default) or ext in ['.Rmd', '.md']:
        return None

    return get_format_implementation(ext).format_name
Return the format name for that extension