Internal header metadata to detect binary endpoints

[JeanJPNM]

Is your feature request related to a problem? Please describe.
#1382 uses the content-type header to check whether the response body is json or binary data, but it doesn't cover all the types that can retrieve binary data, such as image/png and audio/mpeg. Storing metadata inside a custom header or a separate object could make this process much simpler and reliable.

Describe the solution you'd like
A dedicated object to hold internal metadata about the response (such as the type of the body) that is discarded before the response is sent. Such object could be cotained inside a custom header (like x-svelte-meta)or could be an independent entity;

Describe alternatives you've considered
I haven't considered anything else.

How important is this feature to you?
It has a mid level priority for me.

[JeanJPNM]

Closing the issue because it is wrong due to my error while analyzing the code, this issue is irrelevant to the project