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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def guess_content_type(self, pathname):
 """Guess the content type for the given path. :param path: The path of file for which to guess the content type. :return: Returns the content type or ``None`` if the content type could not be determined. Usage: """

        file_basename = os.path.basename(pathname)
        content_type = None

        # Try to determine from the path.
        if not content_type and self._filename_map.has_key(file_basename):
            content_type = self._filename_map[file_basename]
            #logger.debug("Content type of '%s' is '%s' (determined from full "\
            #             "path).", pathname, content_type)

        # Try to determine from the suffix.
        if not content_type and '.' in file_basename:
            extension = "." + file_basename.split(".")[-1]
            extension = extension_case_transform_func(extension)
            try:
                content_type = self._extension_map[extension]
                #logger.debug("Content type of '%s' is '%s' (determined from "\
                #             "suffix '%s').", pathname, content_type, extension)
            except KeyError:
                pass

        # Try to determine from the registered set of regular expression patterns.
        if not content_type:
            for regexp, _content_type in self._regexp_map.iteritems():
                if regexp.search(file_basename):
                    content_type = _content_type
                    #logger.debug(
                    #    "Content type of '%s' is '%s' (matches regexp '%s')",
                    #    pathname, content_type, regexp.pattern)
                    break

        # Try to determine from the file contents.
        if os.path.exists(pathname):
            with open(pathname, 'rb') as f:
                content = f.read()
                if content.startswith("<?xml"):  # cheap XML sniffing
                    content_type = "XML"

        # TODO: Try to determine from mime-type.

        return content_type