Fallback JPG image produces invalid URLs when s3fs is used

Problem/Motivation

In case of using Amazon S3 or Google Cloud Storage for the filesystem, there's an invalid link for the fallback JPG image.

Steps to reproduce

1. install wpf, s3fs
2. set image styles to Convert: WEBP
3. set up a content type's image field to use the Responsive image formatter
4. set up s3fs for using public filesystem as S3 storage.
5. Create a content with image and inspect the fallback image URL.

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes