When I go to upload an image in JPEG XL format, I get the error message "Your post was not accepted because filetype not allowed". Is that something configured by the instance? Is this actually something that I should be taking to the PyLova devs to add as a feature? Should I be taking that to the upstream Piefed devs?

JPEG XL sure seems to be the best image format for many purposes, and software support is growing. I think Chrome and Safari support it and Firefox has experimental support and plans to enable full support by the end of the month. I certainly plan to use the format once it is widely supported.