Prior to this commit, the `ResourceRegionHttpMessageConverter` would
rely on the default implementation of `getDefaultContentType` to guess
the default Content-Type of the resource region to be written to the
HTTP response. That implementation fetches the first media type
provided in the HTTP request "Accept" header.
This behavior is not correct when converting resources and this commits
aligns this converter with the `ResourceHttpMessageConverter` which uses
JAF to guess the correct Content-Type of the given resource, or just
returns "application/octet-stream" as a default value.
Issue: SPR-15041
1ab0916bed