Please submit an IANA request for an official content type
I'd like to be able to request and respond with ION format data, and need an accept header and a content type value for ION data. This shouldn't be hard to add, although getting IANA approval takes quite a while.
Thank you for sharing your interest in an official Ion content type. For the time being, there is an existing provisional media type application/ion that might be suitable for you to use. See https://www.iana.org/assignments/provisional-standard-media-types/provisional-standard-media-types.xhtml
See also https://github.com/amazon-ion/ion-java/issues/82 for the history of this effort. It's something that we've only been able to intermittently work on, unfortunately. If you feel so inclined, we would welcome any contribution that helps move from provisional to finalized status.