Transkribus imports fail with PNG images

I went through the logs of two manual imports I did for @yschneider (Slack thread). Nine images had a cannot write mode RGBA as JPEG error, and all of them have a .png extension. Looking at the code, it seems we only try to convert non-RGB images to RGB when they are not part of the supported image formats, so only when they are not JPG/JP2/PNG, instead of always converting them when img.mode != 'RGB'.