Create a message structure from the data in a file.

def parse(self, fp, headersonly=True):
        """Create a message structure from the data in a file."""
        feedparser = FeedParser(self._class)
        feedparser._set_headersonly()

        try:
            mp = mmap.mmap(fp.fileno(), 0, access=mmap.ACCESS_READ)
        except:
            mp = fp

        data = ""

        # While parsing the header we can convert to us-ascii?
        while True:
            line = mp.readline()
            data = data + line.decode("us-ascii")
            if line == b"\n":
                break
        feedparser.feed(data) # mp[0:5000])
        return feedparser.close()