AIP-143: revisit the naming of "mime_type"

[toumorokoshi]

          I'm not entirely sure. I guess since HTTP calls is content-type, we should follow suit?

https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Content-Type

I don't think we should be constrained by legacy - if there's backwards-incompatible changes we need to introduce let's just provide migration guides. I'd rather minimize the number of suboptimal changes we make for legacy, but open to opinions!

Originally posted by @toumorokoshi in https://github.com/aep-dev/aep.dev/pull/90#discussion_r1486452211