Extract JPEG thumbnail.

        (Thankfully the JPEG data is stored as a unit.)

def extract_jpeg_thumbnail(self):
        """
        Extract JPEG thumbnail.

        (Thankfully the JPEG data is stored as a unit.)
        """
        thumb_offset = self.tags.get('Thumbnail JPEGInterchangeFormat')
        if thumb_offset:
            self.file.seek(self.offset + thumb_offset.values[0])
            size = self.tags['Thumbnail JPEGInterchangeFormatLength'].values[0]
            self.tags['JPEGThumbnail'] = self.file.read(size)

        # Sometimes in a TIFF file, a JPEG thumbnail is hidden in the MakerNote
        # since it's not allowed in a uncompressed TIFF IFD
        if 'JPEGThumbnail' not in self.tags:
            thumb_offset = self.tags.get('MakerNote JPEGThumbnail')
            if thumb_offset:
                self.file.seek(self.offset + thumb_offset.values[0])
                self.tags['JPEGThumbnail'] = self.file.read(thumb_offset.field_length)