Bug Description

On sites with plain permalinks (ie not using rewrite rules) the rest requests in fallback.js result in 404 File Not Found errors.

The cause of the error is that the rest_route always appends the fields portion of the querystring with a question mark, for plain permalinks an ampersand should be used.

Steps to reproduce

1. Configure WordPress to use plain permalinks
2. Upload some a jpeg that will be converted to webp (ie, webp is the smaller file version)
3. Include the image in a post and publish
4. Visit the published post in a browser that does not support webp images
5. In network tools observe that the REST request to get the fallback images fails. The URL will look something like /index.php?rest_route=/wp/v2/media/?_jsonp=wpPerfLab.webpUploadsFallbackWebpImages&per_page=100&include=38
6. Observe the querystring has two question marks.

Screenshots

Additional Context

• PHP Version: 7.4
• OS: Ubuntu?? (What ever exact version Chassis/Chassis uses)
• Browser: macOS/Firefox Dev Edition 104b with image.webp.enabled set to false in about:config
• Plugin Version: trunk at ab08abd
• Device: Laptop