def make_file_list(files):
    """Make a list of subject files.

    Parameters
    ----------
    files : dict
        Collection of files per subject.

    Returns
    -------
    file_lst : list of str
        List of all subject files.
    """

    file_list = []
    for subj, sessions in files.items():
        for session in sessions:
            file_list.append(subj + '_' + session)

    return file_list