This code system http://hl7.org/fhir/content-type defines the following codes:

CodeDisplayDefinition
xmlxmlXML content-type corresponding to the application/fhir+xml mime-type.
jsonjsonJSON content-type corresponding to the application/fhir+json mime-type.
ttlttlRDF content-type corresponding to the text/turtle mime-type.
nonenonePrevent the use of the corresponding http header.