--default-image-extension should probably not apply on encoded data url images

[cderv]

When setting --default-image-extension, it will also apply on already encoded image adding the extension, and creating a bad image.

Probably that extension should not be added when image provided is already a Data URL

This is with Pandoc 3.1.8 using a real data uri for this image

> pandoc -t html
![](data:image/png;base64,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)
^Z
<p><img
src="data:image/png;base64,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" /></p>

# Extension will be added on the data url where it shouldn't to preserve the image
> pandoc -t html --default-image-extension=png
![](data:image/png;base64,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)
^Z
<p><img
src="data:image/png;base64,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.png" /></p>

For context, this was first discussed in quarto-dev/quarto-cli#6568