The PR fixes#39706 by adding `avif` mime type directly to `send`. The PR also removes the previous avif workaround for image optimizer.
Note: The PR is still a workaround for now. I will submit a PR to `pillarjs/send` to help them update `mime` to fix the issue once and for all. But now `send.mime.define` just works.
## Bug
- [x] Related issues linked using `fixes #number`
- [x] Integration tests added
- [ ] Errors have helpful link attached, see `contributing.md`