Update MIME docstring (#56942)
Closes #56768
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Co-authored-by: Steven G. Johnson <stevenj@mit.edu>
Co-authored-by: Max Horn <max@quendi.de>
Co-authored-by: Dilum Aluthge <dilum@aluthge.com>
Co-authored-by: Andy Dienes <51664769+adienes@users.noreply.github.com>
