XData blocks in Caché classes has a MimeType property, and dcoumentation says

MimeType — The MIME type (more formally, the internet media type) of the contents of the XData block. The default is text/xml

But unfortunately I can't set any other values, such as text/html or application/javascript, because XData still expect only XML. 

So, question is, for what reasons this property was appeared?

It would be very usefull to set in this property anything other then text/xml, and Studio would highlighted it as developer expected.