After further investigation, this is what happened.
I had created a default file name for my FileSelectPopup box: "Source.dat" So, when the FileSelectPopup appeared there would already be "Source.dat" in the filename text box. There already existed a file called "source.dat" in the current directory. So, when I pressed OK it was using "source.dat" as my newly selected filename. It assumed that "source.dat" was the same as "Source.dat"
This is not a problem with the FileSelectPopup...it is an OS issue.
Sorry to have posted this. I originally misdiagnosed the problem.
Thanks for trying to help,
Kay