Verify browser-produced WebP files using the container signature
The practical problem
A Canvas export request for WebP can silently fall back to another format depending on browser capability, leaving files mislabeled by an extension alone.
Implementation protocol
Check the Blob type and confirm RIFF bytes at offsets zero through three and WEBP at offsets eight through eleven. Decode the output and inspect transparency and dimensions.
Worked scenario
A valid RIFF/WEBP prefix is useful for format classification, but it cannot by itself prove the entire file is safe or complete.
What could go wrong
Reject unsupported fallback outputs instead of presenting a PNG as WebP.
Acceptance criteria
Test malformed files, JPEG source input and transparent PNG source input.
Source material and evidence boundaries
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