I don't know which of these steps was most effective, but I performed them all and now Win 10 File Explorer: (a) thumbnail previews, & (b) preview pane, are now correctly working for ALL files in a directory
I don't know which of these steps was most effective, but I performed them all and now Win 10 File Explorer: (a) thumbnail previews, & (b) preview pane, are now correctly working for ALL files in a directory