Thumbnail WebP images as WebP

This issue has been migrated from [#14890](https://github.com/matrix-org/synapse/pull/14890).

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I'm testing it since about 3 months (on a single user server) and it works fine.

Closes: https://github.com/matrix-org/synapse/issues/11840
Signed-off-by: Ronny (tastytea) Gutbrod

### Pull Request Checklist

* [x] Pull request is based on the develop branch
* [x] Pull request includes a [changelog file](https://matrix-org.github.io/synapse/latest/development/contributing_guide.html#changelog). The entry should:
- Be a short description of your change which makes sense to users. "Fixed a bug that prevented receiving messages from other servers." instead of "Moved X method from `EventStore` to `EventWorkerStore`.".
- Use markdown where necessary, mostly for `code blocks`.
- End with either a period (.) or an exclamation mark (!).
- Start with a capital letter.
- Feel free to credit yourself, by adding a sentence "Contributed by @github_username." or "Contributed by [Your Name]." to the end of the entry.
* [x] Pull request includes a [sign off](https://matrix-org.github.io/synapse/latest/development/contributing_guide.html#sign-off)
* [x] [Code style](https://matrix-org.github.io/synapse/latest/code_style.html) is correct
(run the [linters](https://matrix-org.github.io/synapse/latest/development/contributing_guide.html#run-the-linters))

Start by reading the migrated pull request #14890 and the superseded issue #11840; no source files or tests are identified in this issue. Confirm the intended thumbnail behavior and determine how completion will verify that WebP images are thumbnailed as WebP, then follow the contributing guide for the changelog, sign-off, and linters.

Written by the indexing model from the issue text.