Allow the webp image format to be supported by default

Current, the webp image format is not supported by default. Including:

• ckeditor not considering webp files as images on drag and drop
• webp file not considered as image by the download action

Technical tasks:

• add webp mimetype to attachment.download.whitelist
• make sure that webp images can be resized server side by the download action

PS: Issue created following this discussion https://forum.xwiki.org/t/how-to-get-xwiki-to-work-with-the-webp-image-file-format/11945

Note: webp is supported by all our supported browsers, see https://caniuse.com/webp