Should compiler guess a content type/accept header if it's absent

[qiaozha]

See this original issue Azure/typespec-azure#1994, in the case where the content type is absent.

  op default(
    @body
    value: bytes,
  ): NoContentResponse;

As discussed in DPG office hour, compiler have two options here:

1. try our best to guess a content type.
2. trust whatever the spec says.

Currently, we lean towards to take option1 for this on the client side. But as this information could be used in both server generation and client generation and maybe other emitters, it would be problematic if the server emitter is taking a different approach than the client emitter.
Maybe the compiler side should unify the logic here?

A proposed guessing logic is:

1. bytes: octect-stream
2. model: json
3. everything else: text/plain

I think this also applies to accept header.