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
InterSystems ObjectScript is a scripting language to operate with data using any data model of InterSystems Data Platform (Objects, Relational, Key-Value, Document, Globals) and to develop business logic for serverside applications on InterSystems Data Platform.