The existing example in rest.txt shows the content property using the html interface.

            "content": {
                "link": "https://.../demo/msg11/"
            },

I think this is ok since we are using the binary_content property for retrieval.

Also text/* is supported for file/msg without a mime type. I'm not sure if there are
security implications for this. Could somebody craft a binary file and set it without
a mime type and do something strange? We set header to block browser sniffing,
so I think it's ok.