Thanks for your reply, Eddy1. Unfortunately, your suggestion does not solve the problem. I inserted the card, found it in File Manager (Nautilus), right-clicked, selected to open it under File Manager (there was no option to set this as a default) and looked at the contents. Then, I ejected the card and reinserted it. Once again, I was presented with a foreground dialog with just two options: open in Shotwell, or eject. My system did not remember my earlier instruction to open it in File Manager.
Clearly, there is some program being too clever by half, in that it assumes I want to look at the photos in Shotwell, even though I have never set such an option as a default. If I mount a file system, I want to control how my system deals with it and will almost always want to use File Manager. How do I find and fix the errant setting?