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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 thumbnail(self):
 """Returns the thumbnail's url for this image, audio, or video file. Returns empty string if the file has no thumbnail"""


        if self.filetype not in ("video", "image", "audio"):
            raise RuntimeError("Only video, audio and image files can have thumbnails")
        thumb_srv = self.thumb.get("server")
        url = f"https://{thumb_srv}" if thumb_srv else None
        return f"{url}/asset/{self.fid}/thumb" if url else ""