Improve display of error returned from the REST API when an unsupported image type is uploaded

[adamsilverstein]

Testing

• Testing requires changes from Error when an (image) upload’s mime type is unsupported wordpress-develop#8322
• Try uploading an AVIF image directly in the block editor

[adamsilverstein]

Unfortunately, this approach leads to a flash of the image showing before the error is received and displayed:

upload.demo.mp4

[github-actions]

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Co-authored-by: adamsilverstein <[email protected]>

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[adamsilverstein]

I have removed the Rest API change here leaving only the improved error handling.

Instead I will add the Rest error directly in core in WordPress/wordpress-develop#8322, the change here improves the error display when the mime type is not supported.

Error before

Error after

Screencast

upload.error.w.gb.mp4

[Mamaduka]

Thanks, @adamsilverstein!