Return content type of file. If file does not
        have a content type, make a guess.

def _get_content_type(self, file):
        """
        Return content type of file. If file does not
        have a content type, make a guess.
        """
        if file.mimetype:
            return file.mimetype

        # get file extension
        _, extension = os.path.splitext(file.name)
        extension = extension.strip('.')

        # Make an educated guess about what the Content-Type should be.
        return media_types[extension] if extension in media_types else 'binary/octet-stream'