If you make a REST GET request for resource metadata against a resource with a filename that includes an extension that is among the supported configuration formats (xml, html, json), then the filename of the resource will determine the content of the response rather than the format parameter.
For example, the request:
Will return metadata about the resource point.xml in XML format with content-type=application/json
However, the request
Will return metadata about the resource point.sld in JSON format, since sld is not a configuration format.