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!