On the web build, {"op":"open","path":…} resolves the name against the
browser's recent-files cache, so a file that is not already there cannot be
opened by the control channel. Accept {"op":"open","name":…,
"data_base64":…} instead: the bytes go through the same parse, caching
and recovery path as a file chosen in the browser picker
(io::open_bytes_web, now shared with pick_and_load_web).
Native builds keep path-based open and reject data_base64 with
invalid_request.
Refs #1349
Helen Kwok
2a78ebe2c1