Why introduce @multipartBody decorator instead of relying only on @body?

[artsiommiksiuk]

From docs (https://typespec.io/docs/libraries/http/multipart/):

import "@typespec/http";

using Http;

op create(
  @header contentType: "multipart/form-data",
  @multipartBody body: {
    username: HttpPart<string>;
    avatar: HttpPart<File>;
  },
): void;

Why having separate @multipartBody, and additionally HttpPart for that? Wouldn't this work?

import "@typespec/http";

using Http;

op create(
  @header contentType: "multipart/form-data",
  @body body: {
    @httpPart("application/json") // override of a part content type
    @encodedName("un")
    username: string;
    // some default behaviour
    avatar: File;
  },
): void;

content-type=form-data should be enough to just treat the same body differently, and HttpPart as a type I think is unnecessary, as you'd be splitting body per root field. You don't have a default fields combination logic anyway. If you want to modify content type of a part it probably should be decorator as a wire specific details, not domain relevant + encodedName to override the serialized name.

All of this reusing existing parts instead of adding new, keeping it leaner.

Or I'm missing something?