svgs not displaying correctly with passthroughImageService

[eya46]

Astro Info

Astro (>=5.15.9)

If this issue only occurs in one browser, which browser is a problem?

No response

Describe the Bug

copy from #14861

After upgrading to the latest Astro (>=5.15.9), all SVG images stopped displaying.

Issues found:

• With passthroughImageService enabled, SVG images return Content-Type: image/null and fail to render (JPG works).

After downgrading Astro, the regression starts at 5.15.9.
Related issues:

• passthroughImageService() does not prevent WebP conversion for Image component in Astro 5.15.2 #14721: images incorrectly converted to WebP
• fix: passthroughImageService generate webp #14776: introduced validateOptions in noop.ts to address it

delete newOptions.format; -> "Content-Type": mime.lookup(format) ?? `image/${format} -> Browser Content-Type: image/null -> image boom (svg)

Changes

I believe the issue in #14721 originates from the options.format = DEFAULT_OUTPUT_FORMAT assignment in service.ts.
To address this, I added a flag to prevent setting a default output format when the noop service is used.

Although my JPG images still return image/null (but browsers can display them), SVG images now work correctly.

before:

after:

What's the expected result?

correctly display svg images

Link to Minimal Reproducible Example

https://stackblitz.com/edit/eya46-astro-svg-bug

Participation

• I am willing to submit a pull request for this issue.