'Return a file-like representation or raise a KeyError.'

def get_file(self, key):

    fname = self._lookup(key)
    if fname.endswith('.gz'):
        f = gzip.open(os.path.join(self._path, fname), 'rb')
    else:
        f = open(os.path.join(self._path, fname), 'rb')
    return mailbox._ProxyFile(f)
