Section 2.1.0.6 Content Types and encodings says that the the _format parameter should support the values xml, text/xml, application/xml (and their json equivalents) and _ interpret _ them as application/xml+fhir (application/json+fhir).

There are two issues requiring clarification:

1. it doesn't say that the Accept header should allow these values, only the _format parameter
2. it doesn't explicitly say that the Content-type header in the response should be application/xml+fhir (application/json+fhir)

I'm seeking clarification that:

1. servers SHOULD also support text/xml, application/xml (and the json equivalents) and interpret them as application/xml+fhir (application/json+fhir), and
2. the Content-type header MUST match the original Accept header / _format paramter value in the case that they are MIME types, but in the case that the _format parameter is xml (json), then the Content-type header MUST be application/xml+fhir (application/json+fhir)

The reasoning behind this is that non-FHIR aware clients are more likely to do something useful when presented with application/xml or text/xml as a Content-type than if they are presented with application/fhir+xml

OTOH, FHIR-aware clients should be able to send an appropriate Accept header or _format parameter if they need a FHIR-specific MIME type to be returned.