def get_files(cls, folder):
        """ Retrieve the list of files the plugin can work on.
        Find this list based on the files name, files extension or even
        actually by reading in the file.

        :arg folder: the path to the folder containing the files to
            check. This folder may contain sub-folders.

        """
        filelist = []
        if folder is None or not os.path.isdir(folder):
            return filelist
        for root, dirs, files in os.walk(folder):
            for filename in files:
                for ext in SUPPORTED_FILES:
                    filename = os.path.join(root, filename)
                    if filename.endswith(ext) and is_excel_file(filename):
                        filelist.append(filename)
        return filelist
Retrieve the list of files the plugin can work on.
        Find this list based on the files name, files extension or even
        actually by reading in the file.

        :arg folder: the path to the folder containing the files to
            check. This folder may contain sub-folders.