Another candidate is screendump. There's no reason to go through a
file. We could pass a pipe as an fd and request a screendump to that
fd, but that's rather roundabout.
Instead of inventing binary syntaxes, maybe we could use dictionary
syntax to denote a mime encoded value?
So something like:
{"Content-Type": "image/ppm", "Content-Transfer-Encoding": "base64",
"Content": "AB234SDFSDf=="}
A special brace format could be used if we wanted to make sure it was
independently parsable:
<"Content-Type": "image/ppm", "Content-Transfer-Encoding": "base64",
"Content": "AB234SDFSDf==">
But I suspect dicts are good enough.