Provide a reasonable default for the subtype field.
Some pre-RFC1521 mailers provide only a type and no subtype in the
Content-Type header field. This function can be used to obtain a
reasonable default subtype given the type of a message. (This is done
automatically by the MIME.Message class.)
Currently, the function uses the following guesses:
"text" | "plain"
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"message" | "rfc822"
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"multipart" | "mixed"
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