in these cases on my opinion we should avoid the common pre part:
´´´ application/vnd.ez.api.ContentTypeGroup+xml ´´´
would become
´´´ ContentTypeGroup+xml ´´´

and for the ```+xml``` should it be separated?
for readability would be best to separate, however the header would need it, so making an header/value and the value incomplete would be kind of messy.