Many Lemmy instances are actually configured so that any time you upload an image file, it gets automatically converted to a WebP, so as to take up less server space. Your instance (lemmy.sdf.org) is the only Lemmy instance that I have experience with that instead actually works in the opposite direction: unless something has changed recently, if you attempt to upload a WebP file to that instance, it automatically gets converted to PNG. This behaviour is all up to how the instance admin chooses to configure their pict-rs backend.