Follow-up to #187: The webp_uploads_content_image_mimes currently controls which image formats can be used as replacements in the frontend. However, the formats that can currently be replaced in the frontend are still hard-coded - at the moment it is just JPEG (specifically its extensions jpg and jpeg).

We need to implement a way so that these source image formats can also be controlled through a filter. For example, if somebody implemented support to create WebP versions for PNG uploads, it should be possible to use the WebP Uploads module's logic to replace PNGs as well.

Related is #237, however let's keep these two decoupled. Depending on the decision in #237, it may simply be that the expected order of the filtered items changes, which would be trivial to update here.