Fixes#18563 by adding the etag header to the optimized image response.
This does _not_ change the expireAt (TTL) for cached files on the server, which still uses the max-age of the upstream response.
The new file format on disk for cached image files is the following:
```
.next/cache/images/<HASHED_QUERYSTRING>/<EXPIREAT>.<ETAG>.<EXT>
```
Older versions of Safari (and other browsers) do not support webp format so we were incorrectly falling back to png.
This PR fixes#18509 so that we fallback to the original image format if a modern format like webp was not explicitly provided in the Accept header.
Tests were added to ensure that the Accept header for Safari, Firefox, and Chrome work properly.
This does two things:
- Rename `iconSizes` to `imageSizes`.
- Give priority to `imageSizes` regardless of `deviceSizes` as a means to opt-out of the srcset behavior.
This separates the `next.config.js` property `images.sizes` into to properties: `images.deviceSizes` and `images.iconSizes`.
The purpose is for images that are not intended to take up the majority of the viewport.
Related to #18122
We currently always accept requests to the new `/_next/image` endpoint, even when it should not be used.
Instead, we should check to see if the default loader is used as a signal to enable this API.
Other loaders (such as cloudinary) will not go through the Next.js API so there is no need to expose this, instead we 404.
- Analogous to https://github.com/vercel/vercel/pull/5321
- Related to #18122
Previously, vector images like svg were being converted to webp and resized.
However, vector images already handle any size so we can bypass the same we do for animated images.
Related to #18122
The `w` parameter in the Image Optimization API is the requested size of the image and should only be resized if the source image is larger than the requested size. This PR fixes the behavior to prevent accidental upscaling, for example icon images.
This updates the new image optimizer endpoint to instead of relying on the `host` and `proto` headers for relative files to use the internal route handling in `next-server` to load files from the public directory. The existing tests for relative files with the endpoint should cover these changes
This makes sure the image optimizer doesn't de-animate images by transforming them with sharp since sharp doesn't currently handle outputting animated images
x-ref: https://github.com/vercel/next.js/pull/17749