def package_contents(self):
        'Returns a dictionary of file information'

        if self.contents_cache:
            return self.contents_cache

        # Get a list of ZipInfo objects.
        files = self.zf.infolist()
        out_files = {}

        # Iterate through each file in the XPI.
        for file_ in files:

            file_doc = {'name': file_.filename,
                        'size': file_.file_size,
                        'name_lower': file_.filename.lower()}

            file_doc['extension'] = file_doc['name_lower'].split('.')[-1]

            out_files[file_.filename] = file_doc

        self.contents_cache = out_files
        return out_files
Returns a dictionary of file information