Copies written by plugins and tools carry their image bytes in the
message: a paint's image.dataBlob indexes the message's blobs. A bare
canvas (what the clipboard holds, see #647) has no images/, so these
fills came in as grey placeholders. The importer now falls back to the
embedded bytes, keyed by image hash, after the archive's images/.
Blobs over the image size cap and indexes past the blobs are skipped and
stay placeholders. Tests: a bare canvas carrying its image in a blob, and
a dataBlob past the blobs.
loyet
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