def METHOD_NAME(files, default_preview=None):
    """Return file to preview or None if no previewable file."""
    selected = None
    for f in files or []:
        file_type = splitext(f.get("key", ""))[1][1:].lower()
        if is_previewable(file_type):
            if selected is None:
                selected = f
            elif f.get("key") == default_preview:
                return f
    return selected