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Content-Type/MIME type is for the format.

There are dedicated headers for other properties, e.g. language.

Actually you're 100% correct.

Feels weird to me that encoding is part of MIME, but language isn't, although I understand why.

Yeah. The reason is that charset is a specific to text types. Language can apply to many media.

Though FWIW, I think the Content-Encoding header is basically a mistake, should should been Content-Transform.