BlobStorage.write() in `auto` compression mode now consults the new
`BlobWriteOptions.mimeType` and skips zstd for MIME types known to be
already heavily compressed — JPEG, PNG, WebP, MP4, WebM, MP3, ZIP, PDF,
Office formats, etc. Gzip/zstd over these formats wastes CPU for no
measurable byte savings; the payload entropy is already near maximal,
so the output is the same size or slightly larger plus the cost of
running the compressor.
The denylist `ALREADY_COMPRESSED_MIME_TYPES` is a conservative set of
well-known formats. False negatives (compressing something we should
have skipped) waste CPU; false positives (skipping something we could
have compressed) waste a few percent of bytes. The denylist favours
CPU-cycle safety because the formats listed here are the ones where
gzip/zstd is reliably a net loss.
The policy applies only to `auto` mode. Explicit `'zstd'` and `'none'`
are honoured because the caller is asserting the choice. The new
`isAlreadyCompressedMimeType()` is exported for consumers that want to
make the same decision before calling `write()`.
VFS / consumer wiring (pass mimeType from VFS through to BlobStorage)
is part of the 2.5.0 #27 storage unification work — when that lands,
every media upload through VFS will engage this policy automatically.
For now consumers opt-in by passing `mimeType` in BlobWriteOptions.
Tests (1447 total, +10):
- isAlreadyCompressedMimeType helper: canonical types, case-insensitive
+ parameter stripping, false-on-missing.
- write() auto-mode: image/jpeg + video/mp4 + application/zip all skip
compression (metadata.compression === 'none'); text/plain is allowed
through to zstd (either 'zstd' or 'none' depending on optional dep).
- Read decompresses transparently regardless of write-side decision.
- Explicit compression options bypass the policy.
