MyPaint currently allows using a non-PNG format, most notably SVG, in its layers. Such a use is not disallowed by the OpenRaster specification for now, but it might not be for long since there’s no mention of other file formats at all in the spec.
To remedy this issue, I have cooked up a clarification/extension proposal (depending on whether they enact PNG-only storage) to formalize the use of non-PNG formats. There are no big code changes required in MyPaint; just adding a MIME type attribute to layer on export will do.
Since I am not a developer of any OpenRaster application, I am very interested in hearing about what a real dev would say. Please comment on my merge request on the spec!
I really like your clarification. From what I know, the openraster specification never really stabilized (it’s still at 0.0.5 last time I checked), so adding requirements for supporting webp/jpg means breaking older readers in some cases. That may be inevitable, but will probably result in a lot of confusion unless those older readers check the ora version field in stack.xml, and enforce a maximum version (with an appropriate explanation to the user).
Since MyPaint is an application with a relatively small userbase, compared to GIMP and Krita, I think it’s more important to have the those devs weigh in on your proposal (iirc, Boudewijn Rempt of Krita came up with the OpenRaster concept in the first place).
