A Download Named WebP May Be PNG: Verify the toBlob Result
Check the requested format, returned Blob and filename separately. A fallback or null callback is not validated by a familiar extension.
An export requests one image format and downloads with its extension, yet the size is unexpected. Inspect the generated Blob and image before repeatedly lowering quality. The request, result and filename need separate evidence.
A format request is not a format guarantee
The MDN toBlob reference states that an omitted or unsupported type falls back to image/png. Browsers must support PNG; other outputs cannot be inferred from a filename. The callback may also receive null when an image cannot be created.
This concerns integrations using that browser API. It does not establish Tooliva's internal implementation or allege an existing fallback fault.
Record four parts of the result
- Keep the source, canvas dimensions and requested MIME string.
- Confirm that a Blob was returned, then record its type and size.
- Inspect actual encoding and dimensions with an appropriate image tool.
- Keep download name, response type and content consistent, retaining the original.
A null result is a failure, not a successful file to fabricate. Renaming returned PNG bytes to .webp does not encode them again. A type field also does not replace content inspection for a file of unknown provenance.
Quality is not a universal size switch
The reference defines quality for formats supporting lossy compression, from zero to one. Omitted or out-of-range values use the user agent's default. It is not a target reduction percentage or a universal PNG lossy setting.
Once encoding is known, compare bytes and appearance. Equal quality numbers do not promise identical output size in every browser.
Review the image after validating its format
Use the color, alpha and compression-quality checklist to protect the delivered asset. Obtaining the requested encoding does not validate visual quality, consumer decoding or texture import.
Hold source and dimensions fixed instead of cropping, resizing and changing format together. Do not invent compression savings where no test has run.
Accept a controlled delivery copy
The Resource Compression tool can prepare a delivery copy for inspection. This does not claim the interface displays Blob.type or exposes every parameter from this API.
Retain requested and actual types, content, dimensions and name. Owners of export code should test the real target browsers. No cross-browser encoding experiment was performed for this article.
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