For some reason I can't connect to the server where the demonstration is stored so I can't comment on the UI, but I can say that I would be interested in that, as I don't feel comfortable using websites like those. I would 100% use an app like this, especially if it were available as a flatpak.
Do you plan to keep it to image/audio conversions, or do you think the app may eventually support other formats (e.g. document formats)?