import zipfile


def unzipper(zip_file, outdir):
    """
    Unzip a given 'zip_file' into the output directory 'outdir'.
    
    Return the names of files in the archive.
    """
    zf = zipfile.ZipFile(zip_file, "r",
                         allowZip64=True)
    filenames = zf.namelist()
    zf.extractall(outdir)
    return filenames