Whenever you click on one of those "Browse" or "Open" or "Save As" buttons, a small Files window will launch and you either look for a file to open up or a place to save one.
Sometimes that functionality disappears, most noticeably in Scratch but also in other applications. The relevant Open or Save button is gone and I can't even overwrite files to override the problem.
I've tried installing other file managers and set them to default but this made no difference.