feat: SVG thumbnails with a fix to the ingredient thumbnail format detection

[brogdonm]

Changes in this pull request

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This adds the ability to correctly list an SVG as a thumbnail type for an asset. This would be useful in the font scenario, where using an SVG representation of the font is more efficient.

Additionally, it was noted that the logic in the Ingredient::update_validation_status and the thumbnail_from_assertion calls were incorrectly assuming the extension would work for a format!("image/{}", ext) call. This was fine for jpg and/or png, but for svg it should be image/svg+xml, so the utils::extension_to_mime method was used in its stead.

Checklist

• This PR represents a single feature, fix, or change.
• All applicable changes have been documented.
• Any TO DO items (or similar) have been entered as GitHub issues and the link to that issue has been included in a comment.