How do i correct a files extension?

I started programming in Python a couple of days ago and I have a problem I couldn't solve yet. I want to correct a files file extension by guessing its mimetype. I tried this:

new_file_name = mimetypes.guess_extension(mimetypes.guess_type开发者_如何转开发(file_name)))
os.rename(file_name, new_file_name)

Why doesnt it work?

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mimetypes uses the existing extension to guess the file type. Use magic instead to examine the contents.