I have a Nikon d7500 also, I have done some test, check the filesize if it's small try to change file extension, from nef to jpg. In my case it is actually a jpg. There is a problem with the settings on Image Quality, I choose NEF(RAW) format from Indi control panel, but it downloads a Jpeg. I really don't know if its a gphoto o indi driver problem. There some Image Quality options that don't read well. I will try to attach some screenshots.