Good question. Let me first give some background to try clarifying why it is as it is ... The "accept" property is intended *only* for use with "raw" encoding of data where Mason has nothing to say about the payload - and as such "schema" isn't supposed to be considered. Accepted formats can also be stated per file for a hypermedia control that allows file uploads (using the "json+files" encoding) - but there is no "schema" property for the files element.
So what happens if multiple formats can be accepted and there is a schema reference? What format does it apply to? I don't know - that was not considered :-)
My best answer is: It is not supported - use only "schema" or "schemaUrl" together with "json" and "json+files" encodings, not with "raw" encoding. In those two encodings your are not supposed to include an "accept" property as this is given strictly by the encoding. The documentation should probably state that "accept" is only valid with "raw" encoding - and "schema(Url)" is only valid with "json" and "json+file" encoding..
But I am open to better solutions :-)