Disclosure: This article was generated with AI for the FrameSprite team, which has a commercial interest in FrameSprite. It received a separate AI editorial review against the cited sources. Executed synthetic fixtures are identified with their runtime and limits; no other editor, engine, production or performance test is implied.
Inspect the actual Blob type before choosing the downloaded sprite's extension. When Canvas does not support a requested encoding format, it can return PNG instead. Calling that result character.gif does not convert its bytes into GIF.
Key takeaway: The requested format, produced media type and filename must agree. A successful encode can still violate the export contract.
Ask for an intentionally unsupported format
The original browser fixture requests image/x-unsupported-fixture from a 1×1 red canvas. The toBlob reference documents PNG fallback when the requested type is not supported.
Chromium 156.0.8078.4 returned image/png, with the signature bytes 137,80,78,71,13,10,26,10. The supported download extension for that observed result is .png, not an extension invented from the requested string.
Choose between accepting fallback and reporting failure
Make the product behavior explicit. If the user asked specifically for GIF, silently delivering a PNG may fail the task even though the image opens. Either use a supported GIF encoder or show that this export path cannot produce the requested format.
If PNG fallback is acceptable, name it honestly:
const blob = await encodeCanvas(canvas, requestedType);
const extensionByType = {
'image/png': '.png',
'image/jpeg': '.jpg',
'image/webp': '.webp'
};
const extension = extensionByType[blob.type];
if (!extension) throw new Error('unsupported produced media type');
const filename = 'character' + extension;
The encodeCanvas helper here means a Promise wrapper that rejects a null toBlob result. Do not pass an unchecked callback result into this naming logic. Also remember that lossy formats or formats without alpha can fail a transparent-sprite requirement even when their extension is correct.
The Canvas serialization specification is the underlying format behavior. The Reference: image-resize-compress maintainer notes document removal of misleading BMP/GIF output claims in that project, which is a useful real-world example of the naming trap rather than evidence about every browser.
Verify bytes at the release boundary
Use the media type for your supported naming policy, then verify the output with an appropriate decoder or signature check before packaging. A type string is metadata; it should not be the sole validation of arbitrary uploaded bytes.
Two assertions ran in the local checker. The result records the actual type, PNG signature and chosen safe extension. The input preserves the deliberately unsupported request. No GIF encoder or engine importer was executed, and no claim is made about GIF support beyond this Canvas path. The HTML checker belongs to the supplied controlled local harness.
Distinguish other misleading downloads
A file containing HTML is a different failure; use the download response-content check. If a PNG begins with a transport compression wrapper, inspect HTTP Content-Encoding separately from PNG compression. For a choice between delivery formats, start with the PNG/WebP/GIF sprite guide.
Keep an unsupported-format negative test in the exporter. The user-visible result should either identify the actual output format or report that the requested format cannot be produced.
Editorial ownership
Prepared by the workflow team, which has a commercial relationship with this site. This guide was drafted with AI assistance; independent editorial review is recorded separately. Report a technical correction with the relevant input, tool version and observed result.
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