Elasticsearch accepts multiple content-type formats, hence scripts can be stored/provided in json, yaml, cbor or smile. Yet the format that should be used internally is json. This is a problem mainly around search templates, as they only support json out of the four content-types, so instead of maintaining the content-type of the request we should rather convert the scripts/templates to json.
Binary formats were not previously supported. If you stored a template in yaml format, you'd get back an error "No encoder found for MIME type [application/yaml]" when trying to execute it. With this commit the request content-type is independent from the template, which always gets converted to json internally. That is transparent to users and doesn't affect the content type of the response obtained when executing the template.
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