For sure it is. But it is not related to my question.
My iPhone is set to HEIC. I want to have HEIC. Why should I want to change this setting to JPEG? Why should I change it back to HEIC since it already IS at HEIC?
The point is that I have HEIC photos in the library, I open a Nextcloud Share in the iPhone browser, hit upload and choose these HEIC photos in my library. And instead of the HEIC version of the file being uploaded, something forces my iPhone to on the fly convert the picture to JPEG and upload a JPEG version to nextcloud. That is what I complain about. I want it to upload the non-converted HEIC versions of my photo library instead.
If you believe this is in any way related to your answer then I indeed fail to see this relation. It looks as if you misread my problem…